1 tablespoon olive oil
1/2 medium onion, finely chopped
8 ounces uncooked chicken or turkey Italian sausage, casings removed
1 (5- to 6-ounce) bag baby spinach
10 large eggs
1 cup whole milk
1/2 teaspoon fine salt, plus more for seasoning
6 (1-ounce) slices cheese, like cheddar, Monterey Jack, or Swiss
6 English muffins, regular or whole-wheat
Arrange a rack in the middle of the oven and heat to 375°F. Meanwhile, grease a 9x13-inch baking dish with cooking spray or oil.
Heat the oil in a large skillet over medium heat. Add the onion, season with salt, and cook, stirring occasionally, until softened, about 5 minutes. Add the sausage and cook, breaking the meat up into small pieces with a wooden spoon, until no longer pink, about 5 minutes. Add the spinach and cook until wilted and the liquid has evaporated, about 4 minutes. Transfer the mixture to the baking dish and spread into an even layer.
Whisk the eggs together with the milk and salt in a large mixing bowl. Pour the eggs into the baking dish. Bake until the eggs are puffed around the edges with golden spots, and a paring knife inserted in the middle comes out clean, 25 to 30 minutes. Cool completely.
While the eggs are cooling, place the English muffins cut-side up on a baking sheet and toast in the oven for just a few minutes, until the edges are toasted. If desired, you can brush the muffins with butter before toasting.
Use a spatula to lift the eggs out of the pan and onto a cutting board. Using a large biscuit cutter or drinking glass (roughly the same size as your English muffins), cut 6 rounds out of the eggs. (Save the scraps to make breakfast quesadillas.)
Lay the bottom halves of the English muffins in a row on a work surface. Top each one with an egg round and a slice of cheese. Finish by adding the muffin tops.
Wrap each sandwich in a square of aluminum foil. Use a permanent marker to write the contents and date on the sandwich. Put all the sandwiches in a freezer bag or airtight container. Freeze for up to 1 month.
Reheating instructions: Unwrap the frozen sandwich and place on a microwave-safe plate lined with a paper towel. (The towel helps absorb some of the melting ice so the bread doesn't get soggy.) Heat at full power for 1 to 1 1/2 minutes, or until the cheese is melted and the frittata is warmed through. (You can also toast the muffin top separately, if you'd like.)
Frozen breakfast sandwiches can also be reheated in a 325°F oven until heated through, about 30 to 40 minutes.
Recipe Notes
- Hot breakfast sandwiches for a crowd: You can serve everything right away. Ass