Preheat the oven to 325ºF and spray a 15”x10” non-stick sheet pan with oil (you can also use 9"x13" casserole dish).
In a large bowl, whisk together the eggs, milk, salt, ground black pepper, and garlic powder. Add the chopped spinach and crumbled feta and mix to combine.
Carefully pour the egg mixture onto the sheet pan. Bake until the eggs are fully cooked, 25 to 30 minutes. Remove the egg bake from the oven and cool completely in the pan.
Once cooled, cut the egg bake into 8 even squares and slice the english muffins in half. Place an egg square in between each muffin. Wrap each sandwich in parchment paper or foil.
Refrigerate in a sealed bag for up to 4 days, or freeze for up to 1 month.
To serve: unwrap the sandwich from the parchment and microwave for about 90 seconds to defrost. For best results, also bake or air-fry at 350ºF until the muffin is toasted and the egg is warm, 3 to 5 minutes.
