Spray a 7-inch non-stick fry pan with cooking spray and heat on medium heat. Add the sliced mushrooms and cook for 2-3 minutes, turning once or twice. Add the fresh spinach and cook for 1-2 minutes or until the spinach has wilted.
Whisk the egg whites in a bowl until light and frothy, add a pinch of kosher salt and pour into the mushroom and spinach mixture. Sprinkle with 1 tablespoon of the parmesan. Let the eggs cook undisturbed until the edges of the egg whites start to cook through and turn a more solid white. Gently lift the edges of the egg whites and tilt the pan so the uncooked egg runs under the cooked part and cook for another minute.
Transfer the fry pan to the oven and broil for 2-3 minutes or until the eggs have puffed and have cooked through. Remove from the oven and sprinkle with the remaining parmesan cheese. Cut into wedges and serve with hot sauce if desired.