Step 1: The All-Important Drain
This is the most critical step for avoiding a soggy sandwich. Open your cans of tuna and drain out all the liquid. For the best results, empty the tuna into a fine-mesh sieve set over a bowl. Use the back of a spoon or a spatula to press down firmly on the tuna, squeezing out as much excess liquid (water or oil) as possible. You should be left with very dry, flaky tuna.

Step 2: Flake the Tuna
Transfer the thoroughly drained tuna to a medium-sized mixing bowl. Use a fork to break up the tuna and flake it apart until it reaches your desired consistency. For a finer-textured salad, flake it thoroughly; for a chunkier salad, leave some larger pieces.

Step 3: Prepare Your Aromatics
Finely dice the celery stalk and finely mince the red onion. The key here is a small, uniform dice. This ensures you get a little bit of crunch and flavor in every bite without being overwhelmed by a large chunk of raw onion or celery. Add the prepared celery and onion to the bowl with the tuna.

Step 4: Add the Flavor Boosters
Add the finely chopped fresh dill, fresh lemon juice, and Dijon mustard to the bowl.

Step 5: Create the Dressing
Add the mayonnaise, black pepper, and salt to the bowl.

Step 6: Gently Fold and Combine
Using a spatula or a spoon, gently fold all the ingredients together until they are just combined. Be careful not to overmix. Overmixing can break down the tuna too much, resulting in a pasty texture. Your goal is a light, cohesive mixture where all the components are evenly distributed but still distinct.

Step 7: Taste and Chill (Optional but Recommended)
Give the tuna salad a taste and adjust the seasoning if necessary. It may need another pinch of salt, a squeeze of lemon, or an extra crack of pepper. For the absolute best flavor, cover the bowl and let the tuna salad chill in the refrigerator for at least 30 minutes. This allows all the flavors to meld and deepen.

Step 8: Assemble Your Masterpiece
When you’re ready to serve, gather your bread. You can use it fresh or lightly toasted. Divide the tuna salad mixture evenly among four slices of bread. Top with crisp lettuce leaves and slices of fresh tomato, if desired. Place the remaining four slices of bread on top, slice the sandwiches in half, and serve immediately.