Preheat oven to 350°F (175°C) and grease a 9x9-inch baking dish.
Prepare eggs – In a bowl, whisk together eggs, egg whites, milk, spinach, garlic powder, onion powder, salt, and pepper. Pour into the baking dish.
Bake for 15 to 20 minutes or until eggs are set. Let cool slightly, then slice into four squares.
Toast bagels – Lightly toast in the oven or toaster for better texture.
Assemble sandwiches – Layer a slice of cheese, a turkey slice, and an egg square onto each bagel.
Cool & Wrap – Let cool completely before wrapping each sandwich in parchment paper or foil.
Store – Refrigerate for up to 5 days or freeze for up to 3 months.