Make the Tuna Salad

Drain the canned tuna well, pressing out any extra water. Add the tuna to a bowl and mash into small chunks. Add the mayonnaise, Dijon mustard, chopped celery, pickle relish, red onion, kosher salt and freshly ground black pepper. Mix everything together until well combined.

Make the Tuna Melts

First, slather one side of 4 slices of bread with butter, and heat a large nonstick or cast iron skillet over medium-low heat. Once hot, place two of the buttered sides of the bread down on the heated skillet (if you don't have room for two sandwiches at a time, you'll just need to do this in batches).

Top each slice with a one slice of American cheese then top with a scoop or two of tuna salad, followed by the provolone cheese. Place the other slices of bread (unbuttered sides touching the tuna and cheese).

Gently press down on the sandwiches to encourage toasting. Cover with a lide and cook on one side for a few minutes. Flip the sandwiches and cook a few minutes more, until both sides are golden-brown and the cheese is nice and melty. Slice and serve!