Tuna Melts with Horseradish Mayonnaise

Name: Tuna Melts with Horseradish Mayonnaise
Ingredients: 6 tablespoons mayonnaise; Two 6-ounce cans solid white tuna, drained; 1/4 cup finely chopped celery; 2 scallions, finely chopped; 2 tablespoons finely chopped fresh parsley; 2 teaspoons apple cider vinegar; Kosher salt and freshly ground black pepper; 1 tablespoon whole-grain mustard; 1 tablespoon prepared horseradish; 4 large slices rye bread, lightly toasted; 8 thin slices Havarti cheese; 2 tomatoes, cut into 8 slices; Dill pickles, for serving (optional)