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

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 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

1. In a bowl, combine the tuna, mayo, onion, relish, mustard, 1⁄2 teaspoon salt, and 1⁄4 teaspoon pepper and mix until incorporated.

2. Spread one tablespoon of butter on one side of each piece of bread, then flip the slices over so they are buttered-side down. Divide the tuna mixture between two slices of bread, then top with the tomato slices and cheddar. Place the remaining two slices of bread on top, buttered-side up.

3. Heat a heavy skillet over low heat and toast the sandwiches in it until the cheese is melted and the bread is golden and toasty, five to six minutes per side.