In a small bowl, whisk together the egg and egg whites. Heat the oil in an 8-inch nonstick skillet over medium-low heat. Add the scallions, tomato, and mushrooms. Cook, stirring, until the mushrooms are tender, 2 minutes. Pour in the egg mixture and cook 2 minutes. Using a rubber spatula, fold the egg in half and cook 1 minute more or until set. Transfer to a plate.