This caramelized onion, mushroom and spinach frittata is savory, fluffy, and loaded with flavor from sweet caramelized onions, tender mushrooms, fresh spinach, and nutty Gruyere cheese. It is the kind of breakfast or brunch recipe that feels a little fancy, but is still made with simple ingredients in one skillet.
Frittatas are basically crustless quiches, which means you get all the flavor and richness without dealing with pie dough. Serve this one for brunch, slice it up for meal prep, or pair it with a simple salad for an easy light dinner.
After making this a few times, the two things that make the biggest difference are giving the onions enough time to caramelize and pulling the frittata from the oven before it looks overdone. Those two small details keep it flavorful, tender, and not rubbery. The onions take a little patience, but that is where the magic happens.
Why You’ll Love This Frittata
This frittata is hearty enough for brunch but simple enough for a weekday breakfast.
The combination of caramelized onions, mushrooms, spinach, and Gruyere gives it plenty of flavor without needing a long ingredient list.
It is also flexible. Serve it warm from the skillet, at room temperature for brunch, or reheated the next day when you want breakfast to feel like you made an effort.
Tips for the Best Frittata
Use full-fat dairy
A little dairy helps keep the eggs tender and creamy. Half and half works well, but whole milk with a splash of cream is also a good option. Avoid low-fat dairy if you can because it does not give the same soft texture.
Get the right ratio
Frittatas are simple, but proportions matter. For every dozen eggs, use about ½ cup of dairy. Too much dairy can make the eggs loose and difficult to set, while too little can make the frittata taste dense and overly eggy.
Do not rush the onions
The caramelized onions are the biggest flavor builder in this recipe. Cook them low and slow until they are soft, golden, and sweet. If they start browning too fast, lower the heat. I know 30 to 40 minutes sounds long for onions, but this is where most of the flavor comes from. If you rush this step, the onions will be soft, but they will not have that sweet, golden flavor that makes the frittata taste special.
Use the right skillet
A 10-inch oven-safe skillet gives this 12-egg frittata enough room to cook evenly while still keeping some height. A larger skillet will work, but the frittata will be thinner.
Cook out the mushroom moisture
One mistake to avoid is adding the eggs before the mushroom liquid has cooked off. Mushrooms release more moisture than you expect, and that extra liquid can make the center of the frittata watery.
Do not overbake it
I like to take the frittata out when the center is just set and still looks slightly soft. It will keep cooking in the hot skillet as it rests. If you wait until it looks completely firm in the oven, it can end up dry.
What to Serve with Frittata
This frittata works well with a simple green salad, fresh fruit, toast, roasted potatoes, or crispy home fries. For brunch, serve it with muffins, coffee cake, or a fresh fruit platter.
Storage and Reheating
Store leftover frittata in an airtight container in the refrigerator for up to 4 days.
Reheat slices gently in the microwave, covered, until just warmed through. You can also reheat them in a low oven. Avoid overheating, or the eggs can become rubbery.
Make-Ahead Tips
You can caramelize the onions and cook the mushrooms ahead of time. Store them in the refrigerator, then assemble and bake the frittata when you are ready.
You can also bake the full frittata ahead and reheat individual slices for breakfast or brunch.
Caramelized Onion, Mushroom and Spinach Frittata
- Total Time: 1 hr 20 min
- Yield: 10-inch frittata, 8 servings 1x
- Diet: Vegetarian
Description
This caramelized onion, mushroom and spinach frittata is filled with sweet caramelized onions, tender mushrooms, fresh baby spinach, and Gruyere cheese. It is an easy, savory recipe for breakfast, brunch, meal prep, or a light dinner.
Ingredients
- 12 large eggs
- ½ cup half and half, or whole milk with a splash of heavy cream
- 1 large onion, sliced
- 8 ounces baby bella mushrooms, sliced
- 2 to 3 cups fresh baby spinach, stems removed and roughly chopped
- 2 cups shredded Gruyere cheese
- 2 tablespoons salted butter, divided
- 2 tablespoons olive oil, divided
- 1 teaspoon sea salt, plust more for the onions
- ½ teaspoon freshly ground black pepper
- ½ teaspoon paprika
- ½ teaspoon dried oregano
- ½ teaspoon dried thyme
- ⅛ to ¼ teaspoon red pepper flakes, optional
Instructions
- Preheat the oven to 375°F.
- Heat 1 tablespoon olive oil and 1 tablespoon butter in a 10-inch oven-safe skillet over medium-low heat. Add the sliced onion and season lightly with salt. Cook, stirring often, for 35 to 40 minutes, until the onion is soft, golden, and deeply caramelized. If the onions start browning too quickly, lower the heat. Remove ¼ cup of the caramelized onions and set aside for topping.
- Add the remaining 1 tablespoon olive oil and 1 tablespoon butter to the skillet, if needed. Add the sliced mushrooms and cook for about 5 minutes, or until they have released their liquid and the liquid has evaporated.
- Add the chopped spinach and cook for about 1 minute, just until wilted.
- In a large bowl, whisk together the eggs, half and half, sea salt, black pepper, paprika, oregano, thyme, and red pepper flakes, if using. Whisk until the mixture is smooth and slightly frothy.
- Stir the shredded Gruyere into the egg mixture.
- Pour the egg mixture into the skillet over the vegetables. Gently move the filling around with a spatula if needed so the onions, mushrooms, and spinach are evenly distributed. Top with the reserved caramelized onions.
- Transfer the skillet to the middle rack of the oven. Bake for 22 to 25 minutes, or until the center is just set. Do not overbake. The frittata will continue to cook slightly as it rests.
- Let the frittata cool for 10 minutes before slicing. Serve warm, at room temperature, or with sour cream, avocado, or fresh chives.
Notes
- A 10-inch oven-safe skillet works best for this recipe. A larger skillet will make a thinner frittata and may reduce the bake time.
- For a potluck or brunch, the frittata can be baked ahead and served warm or at room temperature.
- If baking in a disposable aluminum pan, cook the onions, mushrooms, and spinach in a skillet first, then transfer them to a greased 9×13-inch aluminum pan. Add the egg mixture and bake until the center is set. The frittata will be thinner, so begin checking earlier.
- Prep Time: 10 min
- Cook Time: 1 hr 10 min
- Category: Breakfast
- Method: Baking, Stovetop
- Cuisine: American
Other Breakfast Favorites to Try
If you are planning a breakfast or brunch spread, try one of these next:
- Homemade Blueberry Muffins
- Egg Stuffed Breakfast Peppers
- Lemon Poppy Seed Muffins
- Spinach and Gruyere Quiche with Hash Brown Crust
CARAMELIZED ONION, MUSHROOM, SPINACH AND GRUYERE FRITTATA
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Wow, this was SO flavorful!! Followed the recipe exactly and it came out beautifully. Definitely a keeper.
Hi! I am thinking of making this for a potluck brunch and was wondering if it would work well to make it one of those aluminum pans (instead of a skillet, which would be difficult to transport)! Also, how many servings does the recipe yield? Thank you!
I agree with others. This was a very well made frittata. I like Debi’s explanation of ratios, most food blogs don’t tell you things like that. She explains things well. Great recipe.
Made this for a brunch with my girlfriends and it was a hit! People went in for seconds. Absolutely delicious! And super easy to make. The most difficult/time-consuming part of the process was caramelizing the onions, and even that wasn’t hard. Will be making this again for sure.
I’m so glad to hear it Jenna!
What’s the serving amount for this recipe?
Wanted a frittata recipe instead of a quiche. Boy am I glad I did. Excellent, easy recipe that’s delicious.
Will be back to site, as I liked the design and layout of recipes on her blog.
Thank you!
thanks Doug. I hope you find many recipes that you like.