Preheat the oven to 350°F (175°C). Coat a 9x9-inch square baking dish with non-stick cooking spray.
In a large mixing bowl, whisk together the egg whites, whole eggs, cottage cheese, salt, and pepper until the mixture is well combined and the cottage cheese curds are evenly distributed.
Fold the chopped baby spinach into the egg mixture.
Pour the egg mixture into the prepared baking dish, ensuring the spinach is spread evenly across the surface.
Bake for 18 to 20 minutes, or until the center is set and a toothpick inserted into the middle comes out clean.
Remove the dish from the oven and let it cool for 5 minutes. Slice the egg bake into 4 equal squares.
While the eggs cool, toast the English muffins until they reach the desired level of crispness.
Assemble each sandwich by placing one egg square on the bottom half of an English muffin.
Top the egg with one slice of cheddar cheese and the top half of the muffin.
Serve immediately or wrap individual sandwiches in parchment paper for refrigerated storage.