In a mixing bowl, combine the chopped hard-boiled eggs, Greek yogurt, Dijon mustard, lemon juice, salt, and pepper. Mix well until everything is evenly combined.
Add the chopped celery and red onion to the egg mixture. Stir until they are evenly distributed.
Taste and adjust seasoning if needed.
To assemble the sandwich, scoop the egg salad onto whole grain bread slices or lettuce wraps.
If using bread, you can add lettuce leaves, tomato slices, or cucumber slices on top of the egg salad before closing the sandwich.