Preheat the oven to 350ºF. In a large bowl, whisk together the eggs, milk, salt, and pepper.
Thaw the spinach (I used the microwave), then squeeze out the excess moisture. Take the peppers out of the liquid in the jar, then slice into thin strips. Cut the pepper strips crosswise into smaller pieces. Stir the spinach and peppers into the egg mixture.
Coat an 8x9 or 8x12 inch casserole dish with nonstick spray. Pour the egg and vegetable mixture into the dish. Bake the eggs for about 30 minutes, or until the center is set and the outer edges are slightly browned. Allow the eggs to cool, then slice into six pieces.
Build the sandwiches by adding one piece of the baked eggs and one slice of cheese to each English muffin. Wrap each sandwich in plastic wrap or a fold top sandwich bag, then place all the sandwiches in a gallon sized freezer bag. Freeze for up to 2 months.
To reheat, unwrap the sandwich, place on a microwave safe plate, then microwave on the defrost setting for one minute. Following the defrost, heat on high for 30 seconds at a time until heated through. Or, thaw in the refrigerator over night and simply microwave on high until heated through.