Drain both cans of tuna thoroughly and add to a medium mixing bowl.
Flake the tuna apart with a fork until no large chunks remain.
Add mayonnaise, Dijon mustard, diced celery, and red onion to the bowl. Squeeze in the lemon juice, then season generously with salt and pepper.
Stir everything together until well combined. Taste and adjust seasoning as needed.
Toast your bread if desired, then spread a thin layer of mayo on each slice.
Pile on the tuna mixture, then layer with lettuce.
Slice in half and serve immediately.