Classic Tuna Melts

Name: Classic Tuna Melts
Ingredients: 3 tablespoons butter, softened; 3 large (sandwich-size) English muffins, halved; Two 12-ounce cans solid white tuna, drained; One 12-ounce jar marinated artichoke hearts, drained and chopped; 1 cup mayonnaise; 1/2 cup pitted kalamata olives, chopped; 1 tablespoon whole-grain mustard; 1 tablespoon chopped fresh dill; Several dashes hot sauce; 3 stalks celery, finely chopped; 2 scallions, finely chopped; Zest of 1/2 lemon; Kosher salt and freshly ground black pepper; 12 slices Swiss cheese; 1 Roma tomato, sliced; 2 teaspoons chopped chives