In a heavy nonstick skillet, saute the onion with some of the olive oil or cooking spray over medium heat for 1 minute. Add the green onion, garlic, tomato and serrano pepper and cook until the tomato has rendered most of its moisture. Beat the eggs and whites together and add to the sauteed onion mixture, stirring gently. When the eggs are half set, add the tortilla pieces and continue to cook until almost set. Fold in the cheese and cilantro and season, to taste, with salt and pepper.