Preheat oven to 350 degrees F. Spray 8- x 8-inch pan with non-stick cooking spray.
Sprinkle green onions, chopped spinach, and mushrooms onto the bottom of prepared baking pan. Let cook in the oven for 5 minutes.
While the vegetables are cooking, pour the egg whites in a bowl. Add salt, pepper, and crumbled cooked bacon to the egg whites and whisk together.
Remove pan (with the vegetables) from oven. Lightly spray the tops with non-stick cooking spray and then pour egg white mixture over the cooked vegetables. Bake in oven approximately 15 minutes or until the internal temperature registers 160 degrees F.
Remove from oven and let cool for 30 minutes before cutting into six layers.
I then wrap and refrigerate the pre-sliced frittata. Use wax paper between each piece and layer in stacks of alternating wax paper and cooked frittata. Seal in an airtight storage bag in the refrigerator.
When ready to make your breakfast sandwich, toast bagel or English muffin.
Remove prepared egg white frittata from refrigerator and the wax paper and place on plate. Heat in microwave the for 15 seconds, turn over and heat for an additional 15 seconds. Place slice of cheese (if desired) on cooked frittata and nestle the frittata in between the slices of a toasted bagel/English muffin. Add your choice of tomato, avocado, or fresh spinach.
Makes 6 servings.