There is something deeply satisfying about pulling a golden, bubbling egg bake from the oven — the edges bronzed and crisp, the center still trembling with custard softness, and the whole kitchen smelling of browned butter and sage. This Italian-inspired bake pairs earthy cremini mushrooms with the nutty, melt-in-your-mouth richness of fontina cheese, all bound together in a savory egg custard that slices cleanly and reheats beautifully for days. A scatter of whole sage leaves, fried until they shatter at a touch, turns each serving into something that feels far more elegant than the effort involved.

With 49 grams of fat and only 5 grams of net carbs per generous slice, this bake sits squarely in the keto sweet spot. The eggs and fontina deliver 30 grams of protein per serving, making this a complete dinner that keeps you full through the evening without any need for sides. Heavy cream enriches the custard base and drives the fat ratio to about 75 percent of total calories — exactly where a well-formulated ketogenic meal should land.

The real magic here is the meal-prep angle. One 9x13-inch pan gives you four substantial dinner portions. Bake it on Sunday, let it cool, slice into quarters, and refrigerate. Each piece reheats in the microwave in about two minutes or in a 350°F oven in ten, holding its texture remarkably well through day five. Pair it with a simple green salad dressed in olive oil and lemon, and you have a complete weeknight dinner with zero decision fatigue.

Ingredients (serves 4)

For the mushroom filling:

  • 3 tablespoons (42g) unsalted butter, divided
  • 8 ounces (225g) cremini mushrooms, sliced 1/4-inch thick
  • 3 cups (90g) baby spinach
  • 3 cloves garlic, minced
  • 1/4 teaspoon freshly grated nutmeg
  • 1/2 teaspoon fine sea salt
  • 1/4 teaspoon black pepper

For the egg custard:

  • 10 large eggs
  • 1/2 cup (120ml) heavy whipping cream
  • 6 ounces (170g) fontina cheese, shredded (about 1 1/2 cups)
  • 1 ounce (28g) Parmigiano-Reggiano, finely grated (about 1/3 cup)

For the crispy sage topping:

  • 1 tablespoon (14g) unsalted butter
  • 12 fresh sage leaves
  • Flaky sea salt, for finishing

Instructions

  1. Preheat the oven to 375°F (190°C). Grease a 9x13-inch (23x33cm) baking dish with a thin layer of butter or olive oil and set aside.

  2. Cook the mushrooms. Melt 2 tablespoons of butter in a large skillet over medium-high heat. Add the sliced cremini mushrooms in a single layer, resisting the urge to stir for the first 3 minutes — you want deep golden-brown color on the underside. Flip and cook 2 minutes more until tender and richly caramelized. Season with half the salt and pepper.

  3. Wilt the spinach. Push the mushrooms to one side of the skillet. Add the remaining 1 tablespoon of butter to the cleared space. Add the minced garlic and cook for 30 seconds until fragrant, then toss in the baby spinach. Stir constantly for about 1 minute until the spinach is just wilted but still bright green. Sprinkle the nutmeg over everything, stir to combine, and remove the skillet from the heat. Let the mixture cool for 5 minutes while you prepare the custard.

  4. Make the egg custard. Crack all 10 eggs into a large bowl. Add the heavy cream, remaining salt, and remaining pepper. Whisk vigorously for about 30 seconds until the mixture is completely smooth and slightly frothy. Fold in roughly three-quarters of the shredded fontina, reserving the rest for the top.

  5. Assemble the bake. Spread the mushroom and spinach mixture evenly across the bottom of the prepared baking dish. Pour the egg custard over the vegetables, using a fork to gently nudge any mushroom slices that clump together. Scatter the reserved fontina and all the grated Parmigiano-Reggiano across the surface.

  6. Bake until set. Place the dish on the center rack and bake for 28 to 32 minutes. The bake is done when the edges are puffed and golden brown, the center is set but still has a very slight jiggle (it will firm up as it cools), and a knife inserted in the middle comes out clean. If the top is browning too quickly, tent loosely with foil for the final 10 minutes.

  7. Crisp the sage. While the bake is in the oven, melt 1 tablespoon of butter in a small skillet over medium heat. Once the butter foams, add the sage leaves in a single layer. Fry for about 60 to 90 seconds, flipping once, until the leaves are darkened, curled, and crisp but not burnt. Transfer to a paper towel and sprinkle immediately with flaky sea salt.

  8. Rest, top, and slice. Let the bake cool in the dish for at least 10 minutes — this resting period is essential for clean slicing. Arrange the crispy sage leaves over the top, then cut into 4 equal portions. Serve warm.

Nutrition per Serving

Nutrient Amount
Calories ~590 kcal
Fat ~49g
Protein ~30g
Total Carbs ~6g
Fiber ~1g
Net Carbs ~5g

Nutrition is approximate, calculated from the ingredients listed above. Actual values may vary based on specific brands and portion sizes.

Tips & Variations

Store and reheat for up to five days. Let the bake cool completely, then wrap individual portions tightly in plastic wrap or store in airtight containers. Refrigerate for up to 5 days. Reheat individual slices in the microwave for 90 seconds to 2 minutes at 70 percent power for the best texture, or in a 350°F (175°C) oven for 8 to 10 minutes. The bake also freezes well for up to 2 months — thaw overnight in the fridge before reheating.

Swap the cheese to match what you have. Fontina melts beautifully and has a mild, slightly earthy flavor that complements mushrooms perfectly, but gruyère or provolone make excellent substitutes with a similar melt and fat profile. Avoid pre-shredded cheese if possible — the anti-caking starch adds hidden carbs and prevents that smooth, creamy melt.

Double down on the mushroom variety. Replace half the cremini with a mix of shiitake caps and oyster mushrooms for a deeper, more complex flavor. Just be sure to cook them down thoroughly — mushrooms release a lot of moisture, and excess liquid will make the custard soggy. You want the pan nearly dry before transferring to the baking dish.

Watch for hidden carbs in cream. Some heavy cream brands contain added thickeners or sugars that bump the carb count. Check the label and choose a cream with 0 to 1 gram of carbs per tablespoon. Organic heavy whipping cream with a short ingredient list (just cream) is your safest bet.

Turn it into a brunch centerpiece. While this recipe is designed as a hearty dinner, it works beautifully for weekend brunch or a holiday spread. Bake it fresh, let it rest, and serve it straight from the dish with a drizzle of good olive oil and a crack of black pepper. The crispy sage makes it look far more polished than the work involved.

Frequently Asked Questions

Is fontina cheese keto-friendly?
Absolutely. Fontina is one of the best cheeses for keto — it has about 0.4 grams of carbs per ounce, a high fat content of roughly 9 grams per ounce, and it melts into a smooth, creamy layer without separating or becoming greasy. It is a semi-soft Italian cheese made from cow's milk, and its mild, slightly nutty flavor pairs naturally with eggs without overpowering the other ingredients. If you cannot find fontina at your grocery store, look for it at the deli counter or substitute with young gouda or gruyère, which have a similar fat-to-carb profile.
Can I use a different vegetable instead of mushrooms?
Yes, but choose low-carb vegetables that release minimal water during cooking. Diced zucchini (salted and squeezed dry first), roasted bell pepper strips, or artichoke hearts all work well and keep the net carbs in a similar range. Avoid onions as a primary vegetable — while a small amount is fine, a full substitution would add significant carbs. Whatever you use, cook the vegetables until most of their moisture has evaporated before assembling the bake. Excess water will prevent the custard from setting properly and create a soggy bottom.
How do I store and reheat this for weekly meal prep?
This bake was designed specifically for meal prep and holds up exceptionally well. After baking, let it cool completely at room temperature for about 30 minutes, then slice into portions and transfer to individual airtight containers. Refrigerate for up to 5 days. For the best reheated texture, use the microwave at 70 percent power for 90 seconds to 2 minutes — full power tends to make the eggs rubbery around the edges. If you prefer oven reheating, place the slice on a small baking sheet and warm at 350°F for 8 to 10 minutes. The crispy sage will soften during storage, so if presentation matters, fry a few fresh leaves when you reheat.
Can I make this dairy-free or vegan keto?
A fully vegan version would require replacing the eggs, cream, and both cheeses, which fundamentally changes this dish. However, you can make it dairy-free while keeping the eggs by using full-fat coconut cream instead of heavy cream and a dairy-free cheese that melts well, such as Violife or Miyoko's. Sprinkle nutritional yeast in place of the Parmigiano-Reggiano for that savory, umami depth. Keep in mind that dairy-free cheeses vary widely in their carb counts — some contain starches that add 2 to 4 grams of carbs per serving, so always check labels and recalculate your macros.
Why did my egg bake turn out watery on the bottom?
The most common cause is undercooking the mushrooms. Cremini mushrooms hold a surprising amount of water, and if they go into the baking dish still releasing liquid, that moisture pools beneath the custard during baking. Make sure you cook the mushrooms over medium-high heat until they are deeply golden and the pan is nearly dry — this takes a full 5 to 6 minutes without stirring too often. The second culprit is overbaking, which causes the egg proteins to contract and squeeze out moisture (called syneresis). Pull the bake from the oven when the center still has a slight jiggle — carryover heat will finish the job during the 10-minute rest.