Whisk together egg whites, eggs, and soy milk. Set aside.
In an oven save skillet, heat olive oil. Add asparagus, spinach, onion, salt, and pepper. Cook on medium heat for 5-7 minutes until asparagus is tender and spinach has wilted.
Pour the egg mixture over the veggies and mix in about half of the feta. Cook for an additional 5-7 minutes on medium-low heat until the eggs are mostly set.
Sprinkle the remaining feta over the eggs and transfer the skillet to the oven. Bake for 10-15 minutes until frittata is firm. Broil for 1-2 minutes to brown cheese.