With a fork, beat egg, egg whites, and 1 tablespoon water to blend.
Set a 6- to 8-inch nonstick frying pan over medium heat. When the pan is hot, add the egg mixture. As mixture sets, use a wide spatula to lift up the cooked portion and let liquid flow underneath until the eggs are cooked as you like.
Pour cooked eggs into a bowl and break into bite-size pieces with a fork. Let stand to cool slightly.
Mix mayonnaise, mustard, olives, and onion. Add mixture to eggs and mix. Add salt and pepper to taste.
Spoon egg salad evenly over a bread slice. Top with alfalfa sprouts and the remaining slice of bread.
Note: If you use purchased egg substitute (1/2 cup) skip step