Step 1: Prepare the Tuna
Drain the canned tuna thoroughly, pressing out any extra water to avoid a soggy salad. Place the drained tuna in a mixing bowl and use a fork to mash into small chunks, ensuring a uniform texture.
Step 2: Mix Ingredients
Add the mayonnaise, Dijon mustard, chopped celery, pickle relish, red onion, kosher salt, and freshly ground black pepper to the bowl. Stir thoroughly until all components are evenly mixed, achieving a creamy consistency with evenly distributed crunch.
Step 3: Assemble the Sandwiches
Toast the bread slices until golden. For extra flavor, spread with butter or mayonnaise. Layer a slice of toast with fresh lettuce, followed by half of the tuna salad. Top with another toast slice. Repeat the process for the remaining bread and tuna. Cut sandwiches in half before serving.