Preheat oven to 325 and spray a 15"x10" non-stick sheet pan with oil (you can also use a casserole dish or deep baking dish).
In a mixing bowl, whisk together the eggs, milk, salt, black pepper, and garlic powder.
Add the chopped spinach and crumbled feta and mix with a spatula to incorporate.
Carefully pour the egg mixture onto the sheet pan and evenly spread it out.
Bake for 25-30 minutes, until the eggs are fully cooked.
Remove from oven and let cool completely.
Cut the eggs into 8 even squares, slice the english muffins in half, and place an egg square in between each one.
Wrap each sandwich in parchment paper and store in a large freezer-safe bag.
Freeze for up to one month.
To reheat, unwrap from the parchment and place on a microwave-safe dish.
Microwave for 90-120 seconds, and if desired place in the airfryer or oven at 350 degrees for 3-5 minutes to toast the english muffin.