Put the zucchini onto a double layer of paper towels. Cover with another double layer of paper towels and press out the excess moisture. Coat a small nonstick skillet with cooking spray and place over medium-high heat. Add the drained zucchini, onion, and mushrooms to the pan, season with salt and pepper, and cook until browned and soft, about 5 to 7 minutes. Add the tomatoes and Parmesan and place the mixture into a small bowl. Wipe the nonstick pan clean, coat again with cooking spray, and place over medium-high heat. When the pan is good and hot, add the 3 egg whites and turn the skillet to evenly coat the bottom. When just about set, add the vegetable mixture to one side of the omelet. Gently fold the other side over the vegetables to form a half moon. Carefully slide the omelet onto a heated plate. Serve immediately with a sprinkling of fresh black pepper and the parsley, if using.