Instructions

• Drain the 16 ounces of tuna very well and place it in a medium bowl. Add 1/4 cup mayonnaise, 1/4 cup finely chopped celery, 1/4 cup finely chopped red onion, 1 tablespoon Dijon mustard, 2 tablespoons chopped parsley, and salt and pepper to taste. Stir until evenly combined; set the tuna mixture aside.
• Lay out the 16 slices of bread. Spread the 1/4 cup softened butter evenly on one side of each slice.
• Assemble 8 sandwiches: for each sandwich, place one slice of bread buttered side down. Put 1 slice of cheddar cheese (use the 8 slices total, one per sandwich) on the unbuttered side, divide the tuna mixture evenly among the 8 sandwiches and spread it over the cheese, add tomato slices as desired, then top with the remaining bread slice buttered side up. (If you have extra cheese and want a second layer under the tuna, add it now.)
• Skillet cooking method (recommended): Preheat a large skillet over medium heat. When hot, place 2–4 sandwiches in the skillet (do not overcrowd). Cook 2–3 minutes on the first side, until the bread is golden brown. Flip each sandwich, reduce heat to medium-low, cover the skillet with a lid (to help the cheese melt), and cook 2–3 more minutes until the cheese is fully melted and the second side is golden. Remove and let rest 1 minute before serving. Repeat with remaining sandwiches.
• Broiler cooking method (alternative): Preheat your broiler and position the oven rack about 6 inches from the heat source. Place assembled sandwiches on a baking sheet. Broil 2–3 minutes on the top side until the bread is golden and the cheese begins to melt, then carefully flip each sandwich and broil another 2–3 minutes until the cheese is fully melted and the bread is golden. Watch closely to prevent burning.
• Slice sandwiches if desired and serve hot.