Croissant Breakfast Casserole

Breakfast casseroles are an easy way to take the fuss out of breakfast or brunch, and this Croissant Breakfast Casserole made with flaky croissants, ham, spinach, and cheese is just about as easy and tasty as they get. Prep it the night before, pop it in the fridge overnight, then bake and serve. Brunch is ready!

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I love breakfast casseroles and make this particular one pretty often. I love the combination of croissants, ham, cheese, and spinach, and it all cooks up hot and hearty and melty and delicious.

My favorite part about this Croissant Breakfast Casserole is that you prep it the night before and leave it in the fridge overnight. I will often get up a bit early to pop it in the oven in the morning, then go back to bed. Then, about an hour later, brunch is ready. It couldn’t be any easier!

Ingredients

• Cooking oil or butter – I use butter for extra flavor, but avocado or olive oil work just as well.
• Ham – Leftover ham or deli ham are both great options; just dice it into bite-sized pieces.
• Onions – I like sweet onions such as Vidalia or Spanish, but yellow onion is a great substitute.
• Cooked baby spinach – Squeeze out as much moisture as possible so the casserole doesn’t become watery.
• Eggs – Large or extra-large eggs both work well.
• Milk – Whole milk gives the richest custard, but 2% works in a pinch.
• Heavy whipping cream – Half-and-half is a good substitute if you’d like a lighter casserole.
• Dijon mustard – Smooth Dijon works best; don’t substitute yellow mustard, which will change the flavor of the custard.
• Kosher salt and pepper – Season to taste.
• Gruyère cheese – I recommend shredding it yourself for the smoothest melt.
• Green onions – Add them just before baking to keep their fresh flavor.
• Mini croissants – Day-old croissants work best because they soak up the custard without becoming mushy.

How to Make Croissant Breakfast Casserole

Scroll down for the full recipe card with exact measurements and printable instructions.

Spray a 9×13-inch baking dish with nonstick cooking spray and set it aside.

Heat a skillet over medium heat and add the oil or butter. Sauté the onions until they become soft and translucent, stirring occasionally to prevent browning.

Add the ham to the skillet and stir until it is evenly combined with the onions. Cook just until heated through, then remove the skillet from the heat and set it aside.

In a large mixing bowl, beat the eggs until smooth. Whisk in the milk, heavy whipping cream, Dijon mustard, kosher salt, and pepper until fully combined.

Fold the spinach into the egg mixture, stirring until it is evenly distributed.

Arrange the mini croissants in the prepared baking dish, standing them at a slight angle so they overlap. This creates plenty of pockets for the egg mixture to soak into.

Sprinkle the ham and onion mixture evenly over the croissants.

Slowly pour the egg and spinach mixture over the croissants, making sure the liquid reaches all parts of the dish. Gently press the croissants down with the back of a spoon if needed so they absorb some of the custard.

Sprinkle the shredded Gruyère cheese evenly over the top.

Cover the baking dish with foil or plastic wrap and refrigerate overnight, or for at least 8 hours. This resting time allows the croissants to soak up the custard for a rich, tender casserole.

When you’re ready to bake, preheat the oven to 375°F. Cover the casserole with foil and bake for 40 minutes.

Remove the foil and continue baking for 15 to 20 minutes, or until the eggs are set in the center and the top is lightly golden. A knife inserted into the middle should come out clean.

Let the casserole rest for 5 to 10 minutes before serving. This makes it easier to slice and allows the filling to set.

Serve and enjoy!

Chef Jenn’s Tips

• If using large croissants, cut them into quarters before adding them to the baking dish.
• Feel free to add other vegetables, like mushrooms or bell peppers for extra flavor and color.
• Gruyere is my favorite cheese in this breakfast casserole recipe, but you can add any easy-melting cheese. Mozzarella, cheddar, Jack, and Fontina are all good choices.

Make It A Meal

This Croissant Breakfast Casserole is a standout dish on its own, but for a bigger brunch or meal, serve it with freshly baked muffins or crispy bacon for added crunch. Don’t forget a large pot of coffee or a pitcher of mimosas to wash it all down.

Storage

To store leftovers, allow the casserole to cool completely, then cover and refrigerate for up to 3 days. Reheat your leftovers in the oven or microwave until warmed through. To freeze it, wrap the casserole tightly with foil or place it in a freezer-safe container. It can be frozen for up to 2 months. Thaw overnight in the refrigerator before reheating.