Add celery, red onion, and parsley to bowl and stir to combine
Add mayonnaise, mustard and lemon juice to tuna mixture and stir until well combined
Season with salt and pepper to taste.
Butter the outside of the bread slices, layer tuna mixture on the inside of one bread slice, top with cheese and desired toppings, then place another piece of bread, butter side up, on top.
Heat skillet over medium-low heat and cooke until bottom bread is toasted and cheese is warm, flip and cooke on the other side until bread is toasted.
Other Serving Options
Tuna Salad: add desired amount of tuna salad to toast
Cheese: Add cheeses slices and melt in oven or microwave
Tomato: thinly slice tomato and add to top of salad, or on bottom
Sprinkle with salt and pepper if desired
Serve open face or add another slice of toast for a sandwich
Notes
**Mayo - the amount will depend on how creamy you want your tuna salad