Mix the tuna salad. In a bowl, flake the drained tuna with a fork. Stir in mayonnaise, mustard, celery, onion, relish (if using), lemon juice, garlic powder, pepper, and a pinch of salt. Taste — tuna brands vary in saltiness.
Assemble. Butter one side of each bread slice. Place two slices butter-side down on a cutting board. Layer cheese, a generous scoop of tuna salad, optional tomato, more cheese, then top with the remaining bread (butter-side up).
Griddle. Heat a large skillet over medium. Cook sandwiches 3–4 minutes per side, pressing gently with a spatula, until the bread is deep golden and the cheese is fully melted. Lower the heat if the bread browns before the cheese melts.
Rest and slice. Let sit 1 minute, then cut on the diagonal. Serve with pickles.