In a large mixing bowl, combine drained canned tuna, finely chopped celery, red onion, and fresh parsley. Stir in mayonnaise, Dijon mustard, lemon juice, and a pinch of salt and pepper until all ingredients are well mixed and creamy.

Preheat your broiler and line a baking sheet with parchment paper. Butter one side of each bread slice generously before laying them on the prepared sheet. Broil the bread about 4 inches from the heat for 1-2 minutes until golden brown, then flip and toast the other side.

Once the bread is toasted, remove it from the oven and dollop generous spoonfuls of the prepared tuna salad onto each slice. Sprinkle evenly with your choice of cheese.

After the cheese has melted, top each sandwich with slices of fresh tomato. Season lightly with salt and pepper if desired.

If you prefer the baked method, preheat your oven to 400°F. Butter one side of each bread slice and arrange them on a baking sheet with the buttered side facing up. Bake the bread for about 10 minutes.

Once the bread is toasted, spread your creamy tuna salad evenly on top of each slice. Layer your favorite cheese over the salad and return the baking sheet to the oven.