I love breakfast casseroles (I wrote a whole book about casseroles and my favorite chapter was breakfast!). But I feel that too few offer vegetarian options that are hearty enough to stand up to the bacon and sausage-laden classic breakfast dishes. Vegetarian breakfast casseroles often go extra-light, with fresh green vegetables or a smattering of herbs in the eggs. Well, not this one. This is a hearty vegetarian breakfast casserole with all the rich, deep flavor you'd usually get from meat, but even better.
The flavor boost in this easy dish comes from two things: Hot mustard, and sun-dried tomatoes. Spicy mustard adds a piquant flavor — not easily identifiable, but a strong supporting player in the background. (Don't worry — this doesn't taste overwhelmingly of mustard!)
And then there are sun-dried tomatoes, preferably really good ones packed in oil (perhaps with some herbs) to give umami flavor and a little bit of meaty chew here and there. The cheddar is a great flavor in the mix, too. If you're feeling fancy, throw in a few leaves of rosemary or thyme, but it doesn't need it.
This casserole is truly hearty; it's packed with more potatoes than I usually put in a casserole of this size. But the texture is still light and even fluffy, thanks to the shredded potatoes, which don't weigh this down like cubed potatoes can.
It's a great make-ahead breakfast — make a pan of this and take a wedge with you to the office every morning for a few days.
Cheesy Potato Breakfast Casserole with Cheddar & Sun-Dried Tomatoes
Cooking spray or olive oil
1 tablespoon unsalted butter
1 small onion, diced
6 cloves garlic, minced
2/3 cup sun-dried tomatoes, preferably oil-packed, sliced into thin strips
9 large eggs
3 tablespoons spicy mustard
1 cup whole milk
1 1/2 teaspoons kosher salt
1/4 teaspoon freshly ground black pepper
2 cups shredded Cheddar cheese (8 ounces)
6 cups shredded frozen potatoes (1 pound)
Arrange a rack in the middle of the oven and heat to 350°F. Lightly coat a 9x13-inch baking dish with cooking spray or olive oil; set aside.
Heat the butter in a large skillet over medium heat until it melts and foams up. Add the onion and garlic and cook until soft, about 5 minutes. Add the sun-dried tomatoes and cook for about 5 minutes. Remove the skillet from the heat.
Beat the eggs and mustard together in a large bowl. Whisk in the milk, salt, and pepper. Fold in the cheddar cheese, frozen potatoes, and vegetables from the skillet. Transfer the mixture to the prepared baking dish. (At this point, the casserole can be baked immediately or covered and refrigerated for up to 24 hours.)
Bake until the eggs are firm and the top is slightly golden, 35 to 45 minutes. Let cool for 5 minutes before slicing.
Make ahead: The casserole can be assembled, covered, and stored in the refrigerator overnight. Let sit at room temperature for 20 minutes before baking.
Storage: Leftovers will keep for up to 4 days stored in an airtight container in the refrigerator.