Heat oil in a small nonstick skillet over medium heat. Add onion and cook, stirring, until it begins to soften, about 1 minute. Add egg whites, salt and capers (if using) and cook, stirring constantly, until whites are set, about 30 seconds.
To make the sandwich, layer the egg whites, smoked salmon and tomato on English muffin.
1 1/2 starch, 2 lean meat