Prepare the Tuna: Open and drain both cans of tuna thoroughly. Transfer the drained tuna to a medium mixing bowl. Use a fork to flake the tuna into smaller pieces.
Add the Mix-ins: Add the mayonnaise, chopped celery, chopped red onion, sweet pickle relish (if using), Dijon mustard, and lemon juice to the bowl with the tuna.
Combine and Season: Gently stir all the ingredients together until well combined and creamy. Be careful not to overmix, which can make the tuna mushy. Season generously with salt and freshly ground black pepper to your taste. Mix again to distribute the seasonings.
Assemble the Sandwiches: Toast your bread slices if desired. Spread a generous amount of the tuna salad mixture onto two of the bread slices. Top with lettuce leaves and tomato slices if you like.
Complete and Serve: Place the remaining two bread slices on top to complete your sandwiches. Slice in half, if desired, and serve immediately.