Preheat oven to 350 degrees F. Spray a large 13-inch by 9-inch baking dish with cooking spray and set aside.
In a bowl, stir together taco seasoning, red pepper, green onions, and 1 cup of the cheese. Toss together.
In a separate large bowl, beat eggs, egg whites, and milk with a whisk until combined.
Place 3 tortillas in the bottom of your prepared casserole dish, tearing if needed to fit the bottom of the dish.
Top the tortillas with half of the vegetable mixture. Layer 3 more tortillas on top, then top those tortillas with the remaining vegetable mixture. Top that layer with the 3 remaining tortillas. Pour egg mixture over tortillas. Sprinkle remaining 1 cup of cheese on top.
Cover casserole tightly and bake for about 45 minutes, or until the center is set.
Remove the cover and cook for an additional 5-10 minutes to allow the cheese on top to brown and crisp slightly. Serve with salsa or diced tomatoes, sour cream, avocado, or guacamole.