In a large mixing bowl, combine the drained tuna, finely chopped celery, red onion, parsley, mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Mix everything together until well incorporated.

Decide whether you'd like to broil or bake. For broiling, preheat your oven to the broil setting. For baking, set the oven to 400°F.

Butter one side of your chosen bread. For broiling, place buttered bread on a baking sheet and broil for 1-2 minutes until golden.

Spread a generous portion of tuna salad on each toasted slice of bread, covering it entirely.

Top each slice with cheese. If broiling, return to the oven and broil for 2-3 minutes until melted. For baking, flip slices and bake for an additional 5-7 minutes.

Remove from the oven, add tomato slices on top, and season with salt and pepper.

Slice in half if desired and serve warm, possibly with pickle spears or a side salad.