Prepare the Creamy Base — Start by thoroughly draining your tuna until it looks matte and flaky, then combine it with the mayonnaise in a chilled mixing bowl. Fold the mixture gently until the tuna is well-coated and takes on a slight "shimmer." Avoid over-working the fish at this stage; you want to maintain those hearty, distinct flakes rather than creating a smooth, cafeteria-style paste.

Integrate the Aromatics and Acidity — Stir in your finely chopped red onion, celery, and the tablespoon of fresh lemon juice. Watch out: Only stir until the vibrant colors are evenly distributed. If you over-mix now, the salt will begin to draw water out of the celery, which can turn your salad "weepy" or runny before it even hits the bread. Season with salt and pepper only after tasting, as the tuna and mayo often carry enough sodium on their own.

Construct the Moisture Barrier — Lightly toast your bread slices to create a sturdy foundation, or keep them soft for a nostalgic feel. To prevent the bread from becoming a sponge, apply a thin layer of butter or a lettuce leaf first to act as a physical shield. Generously spoon the tuna mixture onto the center of the bread, then top with sliced tomatoes if you desire a juicy, vibrant finish.

Seal and Serve — Close the sandwich and press down very lightly to "set" the filling before slicing it diagonally. Serve immediately while the bread is fresh and the vegetables are at their maximum crunch. This method ensures that every bite offers a perfect contrast between the creamy tuna and the crisp, zesty aromatics.