Start by preparing your tuna. Open the cans of tuna, drain them thoroughly, and place the tuna into a medium mixing bowl. Use a fork to flake the tuna into smaller pieces.

Add the mayonnaise, chopped celery, chopped red onion, Dijon mustard, and lemon juice to the bowl with the tuna. Stir everything together gently until well combined. Be careful not to overmix, as this can make the tuna salad mushy.

Season the tuna salad with salt and freshly ground black pepper. Start with a pinch of each and add more to your preference. Taste and adjust seasonings as needed.

Toast your bread slices to your desired level of crispness. This adds a delightful texture contrast to the creamy tuna salad.

Assemble your sandwiches. Spread a generous amount of the tuna salad mixture onto one slice of toasted bread. If desired, top with lettuce leaves, tomato slices, or pickle chips.

Place the second slice of toasted bread on top to complete your sandwich. Slice in half if preferred, and enjoy immediately.