Overnight Caramelized Onion and Spinach Egg Casserole is an unassumingly delicious breakfast egg bake that will shine on your table for any occasion. 

Caramelized Onion and Spinach Egg Casserole on a white plate with a fork on the side. The plate is on a gray table. The clear casserole dish is in back of the plate.

Who knew that a seemingly standard egg casserole dish could be such a stellar addition to a cozy post-Christmas brunch with friends?  I spent what felt like hours researching egg casserole recipes in the days prior to our brunch and eventually decided to merge two recipes together.

And as it turns out – as if I didn’t already suspect it – caramelized onions, spinach, cheese, and bread in an egg casserole is a fantastic addition to a brunch. Or, you know, any breakfast, where you want to spend time with some of your favorite people. 

A Make Ahead Egg Casserole

The best part about this brunch casserole recipe, aside from it being an outright phenomenal breakfast, is that you’ll make it the night before serving and simply bake it off in the morning.

My make ahead egg casserole is pretty similar in that respect to this recipe and in my opinion, a make ahead casserole is one of the most genius ways to lighten your load when you’re hosting brunch.

Overhead view of Caramelized Onion and Spinach Egg Casserole on a white plate with a fork on the side. The plate is on a gray table. The clear casserole dish is in back of the plate.

Ingredients

This is the basic shopping list for this recipe but be sure to check out the green recipe card below for the full list!

  • day-old bread – baguette, ciabatta, sourdough, country boule, etc.
  • unsalted butter
  • canola oil
  • onions
  • granulated sugar
  • fresh thyme
  • fresh spinach
  • heavy cream
  • milk
  • Dijon mustard
  • large eggs
  • Swiss cheese – regular Swiss, Emmentaler, or Jarlsburg
  • Parmesan cheese
  • Kosher salt and freshly ground black pepper

Can You Use Leftover Caramelized Onions?

Yes, you absolutely can! If you’ve got some leftover caramelized onions in the fridge from another recipe or meal, they’ll go great in this egg casserole. 

This will definitely cut down on your prep time for this recipe too!

Let’s Make this Baked Egg Casserole!

Here’s a quick snapshot of how you’ll make this egg casserole recipe but be sure to check out the full instruction list in the green recipe card below.

STEP 1: Make the caramelized onions. Add the fresh spinach and cook until the spinach is wilted.

STEP 2: Whisk up the egg, milk, and cheese mixture.

STEP 3: Line the bottom of a greased 3-quart casserole dish with bread cubes. Scatter the onion and spinach mixture over the top then pour the egg mixture over the veggies. Sprinkle some extra cheese over the top.

STEP 4: Cover and refrigerate for 8 to 12 hours.

STEP 5: Bake and serve!

Caramelized Onion and Spinach Egg Casserole on a white plate with a fork on the side. The plate is on a gray table. The clear casserole dish is in back of the plate.

Freezing and Reheating an Egg Casserole

It’s super easy to freeze and reheat this breakfast bake! Simply cut the casserole into pieces and wrap the pieces individually in plastic wrap. Then place the wrapped pieces in a zip-top bag and squeeze out all the air to keep the casserole fresh inside.

To reheat, remove however many pieces you want to serve from the freezer the night before you plan to serve them. Place the wrapped pieces in the fridge to thaw overnight.

The next morning, warm the casserole pieces on a plate in the microwave on 50% power for about 1 ½ to 2 minutes.

How Good is this Egg Casserole?

We saw these same friends again a week after said brunch and they were still raving about this dish. Not the homemade waffles or the biscuits and gravy, but this caramelized onion and spinach egg casserole. I think that says it all.

Caramelized Onion and Spinach Egg Casserole on a white plate with a fork on the side. The plate is on a gray table. The clear casserole dish is in back of the plate.

Caramelized Onion and Spinach Egg Casserole

Yield: 8 to 12 servings
Prep Time: 45 minutes
Cook Time: 1 hour 15 minutes
Inactive Time: 8 hours
Total Time: 10 hours

Overnight caramelized onion and spinach egg casserole is a complete winner in my book. Feel free to add other veggies to the mix if you choose - kale, chard, mushrooms, peppers, etc. - and change up the type of cheese you use (fontina would be amazing). It's a versatile recipe that helps you take advantage of doing the hard work of entertaining for a group in advance.

Ingredients

  • 6 cups 1-inch cubed day-old bread (baguette, ciabatta, sourdough - basically whatever you have)
  • 2 tbsp unsalted butter
  • 1 tbsp canola oil
  • 2 large onions, thinly sliced into half-moon slices
  • 1 tsp granulated sugar
  • 2 tbsp minced fresh thyme
  • 12 oz fresh spinach
  • 1 cup heavy cream
  • 2 cups milk
  • 1 tbsp Dijon mustard
  • 10 large eggs
  • 8 oz Emmentaler Swiss cheese (or just plain Swiss if you prefer), grated (about 2 cups), divided
  • 2 oz Parmesan cheese, grated (about 1 cup), divided
  • Kosher salt and freshly ground black pepper

Instructions

  1. Heat a large skillet over medium heat and add the butter and oil. Once the butter has melted, toss in the sliced onions, sugar, thyme, and season with salt and pepper. Cook, stirring occasionally, for 30-35 minutes, until the onions are a deep golden brown.
  2. Add a few handfuls of spinach to the pan on top of the onions and cover the pan. After 2 minutes, start stirring the spinach into the onions with tongs. Cover the pan and cook for another 1 minute; stir the spinach into the onions again. Continue adding, cooking, and stirring until all of the spinach has wilted into the onion mixture. Remove the pan from the heat and allow the mixture to cool to room temperature.
  3. Meanwhile, pour the cream and milk into a large bowl (you can also use your stand mixer or hand mixer for this step). Whisk in the mustard, eggs, ½ teaspoon salt, and ¼ teaspoon pepper until the eggs are well-beaten. Mix in 1 ½ cups of the Emmentaler and ¾ cup of the Parmesan cheeses.
  4. Grease a 9x13-inch (3-quart) baking dish and lay the bread cubes out in the baking dish. Cover the bread with the onion-spinach mixture. Pour the milk-egg-cheese mixture over everything in the baking dish, allowing the wet ingredients to envelope the bread cubes. Sprinkle the remaining cheese over the top. Cover and refrigerate for at least 8 hours - overnight is better.
  5. Preheat the oven to 350° F. Allow the baking dish to sit on the counter while the oven preheats. Place the dish on a baking sheet. Bake until the casserole puffs up, doesn't jiggle in the center when you shake the dish, and is golden brown on top, 1 hour. If the center of the casserole still jiggles after 1 hour, bake for another 10-15 minutes, checking after 8 minutes. Mine took longer than an hour because the dish was so cold from the fridge. If the top starts to brown too much, lay a piece of foil lightly over the top. Allow the casserole to stand for 5 minutes before cutting and serving.

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  • Warm Vanilla Sugar
    February 27, 2014 at 2:59 PM

    I’m so happy you posted this! It’s definitely a brunch winner!

  • Carol at Wild Goose Tea
    March 14, 2014 at 10:03 PM

    This kind of an egg dish is always a favorite of mine because you can make it ahead. Plus if you made too much, eggs like this microwave so well. Good choice on cheese.

    • taraliptak
      March 21, 2014 at 8:29 AM

      You got it, Carol. We had plenty of leftovers (the recipe makes a ton!) and they reheated beautifully in the microwave.

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