Preheat oven to 375 degrees F (190 degrees C). Spray an 8-inch square baking dish with cooking spray.
Heat olive oil in a skillet over medium heat; cook and stir onion, red bell pepper, mushrooms, and asparagus until onion is translucent, 5 to 10 minutes. Spread onion mixture into the prepared baking dish.
Beat egg whites, eggs, and skim milk together in a bowl until smooth; pour over onion mixture. Sprinkle Cheddar cheese over egg mixture.
Bake in the preheated oven until frittata is set in the middle and a fork inserted in the middle comes out clean, 20 to 25 minutes.