Spray a 9x5-inch glass or microwave-safe loaf pan with cooking spray; sprinkle the onion, green bell pepper, and mushrooms into the pan, and toss lightly with a fork just to mix. Season with salt and black pepper, and pour in the egg whites.
Cook in a microwave oven on High setting for 3 minutes. Remove and stir the cooked egg white from the side of the pan into the rest of the ingredients; cook for 3 more minutes on High. If the omelet is still a little runny on top, slice it into chunks and turn them over in the loaf pan; microwave for 30 more seconds on High. Adjust salt and pepper, and serve.