In a bowl, mix tuna, mayonnaise, Dijon mustard, celery, onion, pickles, and season with salt and pepper.
Spread the tuna mixture evenly on 2 slices of bread.
Top each with a slice of cheddar cheese.
Place on a baking sheet and broil on low until the cheese is melted and bubbly (about 2–3 minutes).
Remove from the oven, slice in half, and enjoy.