Heat oven to 350 degrees.
Heat oil in skillet. Add spinach, mushrooms, onions, and dash of salt & pepper. Saute for 3-5 minutes, or until veggies are soft.
In a medium bowl, whisk eggs, egg whites, milk, and another dash of salt & pepper.
Spray 9×13 inch pan with cooking oil. Place cooked veggies evenly in the pan. Pour egg mixture over.
Cook for 16-18 minutes or until eggs are cooked through.
Allow to cool slightly. Cut 6 large circles out of eggs (I used a pumpkin shaped cookie cutter. You could also use a food storage lid or free hand cut with a knife. Or cut into 6 squares and use every last bit!)
Assemble 6 sandwiches using 1 English muffin, 1 slice of cheese, and 1 egg circle for each. Wrap with plastic wrap and place in freezer safe bag. Keep in the freezer for up to a month. Keep in the refrigerator for 3-4 days.
Top with optional toppings, as desired, right before eating and after reheating.
To reheat from frozen, remove from plastic wrap. Tent a paper towel over the sandwich. Microwave for approximately 90 seconds. Allow to cool for a couple minutes before enjoying.