Preheat oven to 375F. Spray a small sheet pan (I used the one that came with my toaster oven it's 8 x 8, you could also just use an 8 x 8 glass baking dish) with cooking spray and/or line with parchment paper.
Whisk eggs and egg whites together in a medium bowl with seasonings, and pour the eggs in the sheet pan.
Saute down the spinach in a little bit of cooking spray in a skillet, then spread evenly across the eggs in the sheet pan. Add in onions evenly across eggs.
Bake for 20-30 minutes until the eggs are set. Once they are set take them out of the oven and let them cool 10 minutes.
Using a mason jar lid create egg disk cut outs for the sandwich. I ate my scraps, you do with them as you wish. For the sandwiches:
Build sandwiches with 1/2 ounce cheese and one egg disk in between English muffin. Wrap with saran wrap, and label them.
TO REHEAT: Unwrap the saran wrap and wrap the sandwich in a paper towel. Microwave 1-2 minutes until defrosted. You can eat as is, but I recommend separating the muffin and putting in the toaster oven on the toast setting (this way it gets both sides.) When you do this one-half of the English muffin will have nothing on it and the other half will have egg on top and cheese on the bottom so you are toasting the top of the egg disk and the bottom of the bread (the cheese is safely housed in the middle.)
Nutrition facts vary depending on type of bread used.