Martha's favorite egg salad sandwich is a healthy—and delicious— lunch choice.
Using a pastry blender or fork, mash together egg whites and yolks in a large bowl; carefully fold in avocado. Stir in mayonnaise, mustard, and lemon juice; season with salt and pepper.
Place 6 slices of bread on a work surface. Evenly divide egg salad among bread slices. Add lettuce greens or tomato, if desired. Top with remaining 6 slices bread. Serve.