Set your oven to 400 F.
Drain the tuna, then transfer it to a mixing bowl
Use a fork to flake apart the tuna.
Add mayonnaise, celery, pickles, onion, and parsley.
Whisk thoroughly.
Prepare the bread by buttering one side.
Spread about ½ cup of tuna salad on 4 slices of bread (butter side down)
Top with two slices of cheese
Place the second piece of bread on top with butter side up.
Repeat on the remaining three sandwiches.
Put the sandwiches on a baking sheet
Bake for about 3-5 minutes, then flip so both sides toast.
Serve when the cheese is melty.