Preheat the oven to 350 degrees F (175 degrees C). Spray a small casserole dish with cooking spray.

Heat oil in a 12-inch nonstick skillet over medium heat. Add bell pepper and onion and cook until tender, about 5 minutes.

While pepper and onion are cooking, combine milk, egg whites, whole eggs, salt, pepper, and cumin into a medium bowl using a whisk.

Transfer cooked vegetables to the prepared casserole dish. Pour egg mixture over the top.

Bake in the preheated oven until eggs are set, about 30 minutes.

Remove frittata from the oven and cut into wedges or squares. Place onto a plate and top with salsa.