Make-Ahead Spinach and Feta Egg Casserole
1 tablespoon olive oil, plus more for the baking dish
1/2 medium yellow onion, diced
1/2 medium red bell pepper, cored, seeded, and diced
1 1/2 teaspoons kosher salt, divided
6 ounces baby spinach (about 6 cups packed), or 8 ounces frozen spinach, thawed, drained, and pressed well to remove liquid
2 cloves garlic, minced
2 tablespoons chopped fresh dill
12 large eggs
2 cups whole or 2% milk
1 tablespoon Dijon mustard
1/4 teaspoon freshly ground black pepper
4 ounces feta cheese, crumbled (about 1 cup)
Arrange a rack in the middle of the oven and heat to 375°F. Coat a 9×13-inch baking dish with olive oil.
Heat the oil in a large frying pan over medium heat until shimmering. Add the onion, pepper, and 1/2 teaspoon of the salt. Cook, stirring occasionally, until softened, about 5 minutes. Add the spinach and garlic, and toss until wilted and almost all of the liquid is evaporated, about 3 minutes. Remove from the heat and stir in the dill. Transfer to the baking dish and spread in an even layer.
Place the eggs, milk, mustard, remaining 1 teaspoon salt, and pepper in a large bowl and whisk until the eggs are completely broken up and incorporated. Pour over the spinach mixture in the baking dish. Sprinkle evenly with the feta.
Bake until the top is light golden-brown and a knife inserted in the middle comes out clean, 40 to 45 minutes. Let cool for 5 minutes before slicing.
Make ahead: The casserole can be assembled, covered tightly with aluminum foil, and refrigerated overnight before baking. Uncover before baking.
Freezing: The casserole can also be frozen unbaked. Cover tightly with aluminum foil, wrap in plastic wrap, and freeze for up to 3 months. Thaw overnight in the refrigerator, then proceed with baking uncovered as instructed. Depending on how frozen the casserole still is, it may take up to 1 1/2 hours baking time; check after 1 hour.
Storage: Leftovers can be stored in an airtight container in the refrigerator for up to 4 days.