In a medium bowl, flake the drained tuna with a fork. Stir in the celery, scallions, and dill. Chop the capers and mix them in along with the lemon juice and mustard.
Add mayonnaise and salt and pepper. Stir well. Taste, and season with more seasonings, if desired.
Toast the four slices of bread in a toaster, and turn on the oven broiler. Position an oven rack in the top third of the oven.
Lay the toasted bread slices on a baking sheet. Divide the tuna salad mixture among the slices of bread, spreading it in an even later. Sprinkle the cheese evenly over the slices then place the sandwiches under the broiler until the cheese is melted. Depending on your broiler, it'll take 1 to 2 minutes, or longer, but keep an eye on them.