Preheat oven to 350, and line a 9 x 13 baking pan with parchment paper.
Finely chop the onion, and saute over medium heat in a large frying pan until tender and starting to brown, 7-10 minutes.
Add the chopped baby spinach and saute gently until spinach is wilted and tender, but not overcooked. Let cool slightly.
In a large mixing bowl, whisk together eggs, cottage cheese, and sauteed veggie mixture.
Pour the egg and veggie mixture into the baking pan and bake at 350 F for 20-22 minutes until eggs are cooked through.
Cut the egg mixture into 12 squares and assemble the sandwiches: one square of eggs and one slice of cheese inside each English muffin.
Wrap in parchment paper and foil to freeze entire sandwich. To reheat, place foil wrapped sandwich in a preheated oven at 350 F for 30 minutes. Or REMOVE FOIL and microwave on high 3 minutes.