Tuna melt

Name: Tuna melt
Cuisine: American
Ingredients: 2 (5-ounce) cans water-packed white tuna, drained; 1/2 cup mayonnaise; 1/3 cup finely diced onion; 2 tablespoons sweet pickle relish; 1 tablespoon Dijon mustard; Kosher salt and freshly ground black pepper; 4 tablespoons (1/2 stick) unsalted butter, at room temperature; 4 slices country white bread; 6 slices vine-ripened tomato; 6 thick or 12 thin cheddar cheese slices