Creamy Caramelized Onion and Mushroom Frittata

By , Test Kitchen Director · Reviewed by AislePrompt Editorial · ·

A protein-rich breakfast frittata with deeply caramelized onions, mushrooms, and a hint of nutmeg for a satisfying morning meal. This american-inspired keto (high protein) ready in about 29 minutes pairs large Eggs, Heavy cream, (thinly sliced) Yellow onions for a weeknight-friendly dinner that comes together with one pan and minimal cleanup. Each serving lands at about 320 calories and feeds 4, so it slots cleanly into a weekly meal plan and pairs well with a quick salad or grain on the side. Tap "Plan a meal with the AI" below to drop this recipe into your week and send the ingredients straight to Instacart. Nutrition values are approximate, calculated from USDA FoodData Central (fdc.nal.usda.gov) — verify against ingredient labels for any health-driven dietary plan.

Prep: 12 min Cook: 17 min Serves 4 American cuisine 320 cal/serving
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Ingredients

Instructions

  1. Step 1: Melt 1 tbsp butter in an oven-safe skillet over medium heat. Add sliced onions and cook for 12 minutes until golden brown, stirring occasionally.
  2. Step 2: Add sliced mushrooms and cook for 5 more minutes until moisture evaporates and mushrooms are browned.
  3. Step 3: Whisk together 6 large eggs, 1/4 cup heavy cream, 1/4 tsp salt, 1/8 tsp black pepper, and 1/4 tsp nutmeg in a bowl.
  4. Step 4: Pour egg mixture over onions and mushrooms in the skillet. Sprinkle with chopped thyme.
  5. Step 5: Transfer skillet to oven and bake at 375°F for 15 minutes until set and golden around the edges.

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Frequently asked questions

How long does Creamy Caramelized Onion and Mushroom Frittata take to make?

Total time is about 29 minutes (12 min prep + 17 min cook). Most home cooks find this fits comfortably into a weeknight; double the batch on Sunday for two dinners.

How do I store leftover Creamy Caramelized Onion and Mushroom Frittata?

Cool to room temperature within 2 hours, then store in an airtight container in the fridge for up to 4 days. Reheat covered in a 350°F oven for 10-12 minutes, or microwave at 70% power in 60-second bursts to keep large eggs from drying out.

Can I substitute ingredients in Creamy Caramelized Onion and Mushroom Frittata?

Yes — most ingredients in this recipe have flexible swaps. For oil, use any neutral high-smoke-point oil (avocado, grapeseed, refined coconut). For aromatics, dried herbs at ⅓ the volume of fresh work in a pinch. The full ingredient list is shoppable on Instacart so you can see substitutes in real time.

How do I scale Creamy Caramelized Onion and Mushroom Frittata for a different number of people?

The recipe is written for 4 servings. Multiply each ingredient by (your serving target / 4). Cook time stays roughly the same up to 2x; for 3-4x batches, switch from a skillet to a sheet pan or stockpot so the food isn't crowded — overcrowding steams instead of browns.

Is Creamy Caramelized Onion and Mushroom Frittata high protein?

Yes — this recipe is tagged high protein based on its ingredient list. Always cross-check labels for packaged ingredients (sauces, broths, condiments) since formulations vary by brand.

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