Caramelized Onion and Zucchini Egg White Casserole is a delicious savory breakfast elegant enough for company, but perfect for meal prep, too.

A green glass casserole dish with zucchini rounds and melted cheese on a placemat.

Caramelized Onions and Zucchini Egg Bake Recipe

For me, breakfast is the best part of the day, and I love a hot, savory breakfast. I also love not having to break out a skillet to cook eggs every morning. An egg casserole is a great option to save time in the morning, but also perfect for feeding company on special occasions.

If you’re avoiding egg yolks, for whatever reason, this healthy breakfast casserole is made only with egg whites. It’s also laden with caramelized onions and fresh zucchini. Melty white cheddar gives it just enough decadence that you’ll want this on repeat.

I love this egg white casserole as a low carb, meal prep option for easy breakfasts. But, I would totally serve it to friends for weekend brunch, too!

A green glass casserole dish with zucchini rounds and melted cheese with a slice missing.

Gather Your Ingredients an Egg White Bake

In addition to olive oil, kosher salt + fresh cracked black pepper, you’ll need…

Small zucchini — look for bright green in color with skin that isn’t wrinkled.
Spanish onion — a large yellow onion with a sweet bent.
Garlic cloves — four to be exact, or use more or less. You can definitely use garlic powder if you happen to be out of fresh.
Calabrian chili paste — optional, but it’s so good!
Dried thyme — or fresh.
Liquid egg whites — the kind that comes in a carton; this is for ease and so as not to waste a dozen perfectly good yolks.
Greek yogurt — provides a little texture and extra protein to the egg whites.
White cheddar cheese — shred it yourself straight off the block. It’ll melt beautifully.

Raw ingredients for caramelized onion and zucchini egg white casserole.

How To Make Caramelized Onion + Zucchini Egg White Casserole

Spread zucchini rounds in a single layer on a sheet pan or cutting board. Sprinkle with a bit of kosher salt, then leave alone while some of their moisture is released.
In a large skillet over medium heat, sauté the onions and garlic with salt + pepper until they are soft and caramelized. Stir in Calabrian chili paste, then transfer to a casserole dish spritzed with cooking spray. Sprinkle on half the cheese.
Blot the zucchini with a towel then add to the skillet with some thyme. Sauté zucchini so it softens and releases more of its moisture. Transfer to the baking dish.
In a large bowl, whisk the egg whites and greek yogurt with salt + pepper until they are foamy. Pour egg mixture over the veggies and sprinkle with remaining cheese.
Bake in a preheated oven until the eggs are set. Rest for 5 minutes before slicing.

A Note on Watery Vegetables: I first salt, then sauté the zucchini to draw out as much of the moisture as possible. Caramelizing the onions has the same effect. If the vegetables aren’t precooked, your casserole will be swimming in liquid. There may still be a tiny bit in the end, but not an offensive amount.

Caramelized strips of onion in a skillet with a wooden spoon resting on the edge.

A green glass casserole dish with caramelized onions and shredded white cheese in it.

A bowl of foamy whipped egg whites and a whisk.

Sprinkling cheese over a casserole dish filled with zucchini rounds and whipped egg whites.

Substitutions + Additions

Spanish onion — feel free to use red onions or a sweet onion like Vidalia.
Calabrian chili paste — I always have a jar in my fridge, but it may be a bit obscure for some. You can omit this ingredient or substitute with red pepper flakes to taste.
Dried thyme — I definitely favor this herb and it does play well with zucchini, but use your favorite dried or fresh herbs instead.
Liquid egg whites — a 16 oz carton of liquid egg whites is just shy of 2 cups. Use whole eggs if you prefer; you’ll need about 10 large eggs to yield just about 2 cups. Use a scale if you’d rather weigh out 16 oz. To crack and separate egg whites yourself, you’ll need 16 to 20 large eggs. Use this reference for additional equivalent measurements. (Use those yolks for other recipes or freeze them for future use.)
White cheddar — I’ve tried this with other cheeses and the white cheddar is my preference, but feta and goat cheese come in a close second.
Greek yogurt — swap this for cottage cheese, milk or cream, if you prefer.
Veggies — swap in or add red bell pepper for another flavor component. I recommend already roasted peppers, or dice and sauté them first.
Extra! — a sprinkling of parmesan cheese can offer a nutty salty bite. Consider scaling back on the salt if you choose to add some parmesan.

What to Serve With This Zucchini Breakfast Casserole

Let’s talk garnishes first…chopped fresh thyme, fresh basil or thinly sliced green onions would all be lovely and add another pop of flavor.

For a simple weekday meal, I’ve been known to wrap a slice of warm casserole in a whole wheat tortilla and call it done. It’s also great with toast, an English muffin or a serving of fresh fruit on the side.

For a brunch, I’d pair it with a simply dressed green salad, whole grain toast, fresh fruit and turkey bacon. With mimosas, of course!

A slice of caramelized onion and zucchini casserole on a white plate next to a half eaten english muffin.

Storing + Reheating

For meal prep, I allow the casserole to cool, cover it with plastic wrap, then pop the whole dish in the fridge. Once a couple servings have been enjoyed, I’ll transfer the rest into an airtight container.

A minute and a half in the microwave is about right for reheating a single slice.

Make Ahead Tips

Option 1: Sauté the onions/garlic and zucchini/thyme per the recipe instructions then allow to cool. Shred the cheese. Store each component in separate airtight containers in the refrigerator. Assemble the casserole while the oven preheats. (Preferred method.)

Option 2: Assemble the entire casserole ahead of time then wrap it tightly to store in the fridge. Let it sit on the counter while the oven preheats. Add a couple more minutes to the bake time if necessary so the egg whites are completely set and hot in the center (160°F).

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