In a large mixing bowl, combine the drained tuna, finely diced celery, minced red onion, and minced parsley. Stir gently to mix the ingredients without breaking the tuna apart too much.
Next, add the mayonnaise, mustard, and lemon juice to the bowl. Start with half a cup of mayonnaise and mix thoroughly. If you prefer a creamier texture, gradually add more mayonnaise until you reach your desired consistency. Season the mixture with salt and pepper to taste.
Heat your skillet or pan over medium heat. Spread a thin layer of butter on one side of each slice of bread. Place the bread butter-side down on the skillet to toast until golden brown.
Once the bread is toasted, flip the slices over. Spoon a generous amount of the tuna mixture onto each slice. Top with cheese slices and fresh tomato slices.
Cover the skillet with a lid and let it cook for a few minutes, allowing the cheese to melt. Keep an eye on it to prevent the bread from burning.
Once the cheese is melted to your liking, remove the tuna melts from the skillet. Let them cool slightly before slicing in half. Serve warm and enjoy your flavorful Keto Tuna Melt!