Using a pastry blender or fork, mash together egg whites and yolks in a large bowl; carefully fold in avocado and relish.
Stir in cottage cheese, mustard, and lemon juice; season with salt and pepper.
Place 3 slices of bread on plates. Evenly divide egg salad among bread slices. Add spinach or tomato, if desired. Top with remaining 3 slices bread and enjoy.
Notes
This is also delicious served on crackers, lettuce wraps or in a tortilla wrap.