Mash eggs with a potato masher in a medium bowl. Add lemon juice, mayonnaise, mustard, salt, pepper, celery, and onion. Mix to combine.
Cover and refrigerate to allow flavours to develop, and to cool filling, about 1 hour.
Serve between slices of bread. Leftovers can be kept refrigerated for up to 3 days.
Notes
Use fresh lemon juice. I don't recommend leaving lemon juice out either. It brings a brightness to the egg salad.
Use whichever type of sandwich bread you prefer. Some recommended ones: multigrain, white, brown, rye. Just make sure to pick a soft bread. If the bread is too hard, the filling will squeeze out the sides.
Hard boil eggs the day before to make this recipe quicker to make.