Preheat the oven to 325ºF. Grease a 9x13-inch baking dish with neutral oil spray and set aside.
Whisk the eggs, milk, garlic powder, onion powder, salt, and black pepper in a large bowl until evenly combined.
Fold in the roasted red peppers, spinach, and cheddar.
Transfer the egg mixture to the prepared baking dish and bake until a toothpick comes out clean, 40 to 45 minutes.
Cool in the dish for 15 minutes then slice into 6 even squares.
Assemble the sandwiches: Place an egg square between each sliced English muffin.
Tightly wrap each sandwich in parchment or foil and refrigerate or freeze.
