Drain the canned solid white albacore tuna thoroughly in a fine-mesh strainer and flake it into a mixing bowl.

In a small bowl, combine Greek yogurt and Dijon mustard. Whisk in lemon juice and sweet pickle relish, then season with salt and pepper.

Add chopped celery, minced red onion, and chopped dill to the flaked tuna. Gently mix with a spatula.

Pour the creamy dressing over the mixture. Fold the ingredients gently to combine without overmixing.

Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld.

Lay fresh lettuce leaves on four slices of bread. Spoon the chilled tuna salad over the lettuce, and layer with tomato slices before topping with the remaining bread.

Lightly toast the bread for better texture, cut the sandwiches in half, and serve with chips or salad.