1. Preheat the oven to 325 degrees F. Spray a 9X13 inch casserole pan with cooking oil and set aside.
2. In a skillet, heat the oil over medium heat.
3. Add the onion and bell pepper, lightly sprinkle with salt and pepper to season, and sauté until softened, about 5 minutes. Remove from heat and allow to cool some.
4. In a medium bowl, whisk together the egg, 1 teaspoon salt, 1/2 teaspoon pepper, and milk. Stir in the onion and bell pepper mixture.
5. Pour the egg mixture into the casserole dish and bake in the preheated oven for 18-25 minutes, until the center is just set.*  (note 2)
6. Allow the eggs to cool before cutting into 12 squares.
7. To assemble the sandwiches, open the English muffins and place an egg square then a slice of cheese on the bottom, and then put the top on.
8. If you are not freezing and eating the sandwiches right away, place the sandwiches back in the oven and heat for 5 minutes, until the cheese is melted.
9. To freeze, wrap the sandwiches in aluminum foil or wax paper, and then place in a freezer-safe resealable bag. Freeze for up to one month.

To Reheat:

1. For best results, thaw a sandwich in the refrigerator overnight.
2. To microwave: remove the foil or wax paper and wrap in a paper towel or clean kitchen towel. Microwave for 40 seconds on 50% power. Turn the sandwich over and microwave another 15-30 seconds on 100% power, until heated through.
3. To reheat in the oven: Reheat at 350 degrees for 10-15 minutes, until the cheese is melted and the sandwich is warm throughout.