Prepare the Vegetables: Wash and prepare all your vegetables as described above. Ensure lettuce leaves are thoroughly dried to prevent a soggy sandwich.
Combine Filling Ingredients: In a medium mixing bowl, flake the drained tuna with a fork. Add the low-fat quark (or Greek yogurt), diced tomatoes, diced cucumber, and chopped fresh dill.
Season the Filling: Gently stir in the fresh lemon juice. Season with the ½ teaspoon of salt and freshly ground black pepper to your taste. Mix gently until all ingredients are just combined – be careful not to overmix, as this can make the tomatoes release too much liquid.
Assemble the Sandwiches: Lay out six slices of the whole wheat bread on a clean work surface. Evenly divide the tuna and tomato mixture among these six slices, spreading it close to the edges.
Add Lettuce and Complete: Neatly arrange two large lettuce leaves on top of the tuna mixture on each slice. Top with the remaining six slices of whole wheat bread.
Serve: For best presentation and easier eating, slice each sandwich diagonally in half with a serrated knife. Serve immediately.