Start by flaking the canned tuna into a large mixing bowl. Add mayonnaise, diced celery, finely chopped red onion, dill relish, Dijon mustard, sliced green onions, lemon juice, kosher salt, and black pepper. Mix everything thoroughly until well combined.

Take your slices of bread and spread softened butter generously on one side of each slice.

On the unbuttered side of four slices of the bread, layer 2 slices of cheddar cheese, followed by 2 slices of juicy tomato. Spread ½ cup of the prepared tuna salad on top of the tomatoes.

Heat a skillet over medium-low heat, allowing it to warm up. Carefully place the prepared sandwiches buttered-side down onto the skillet. Cover with a lid and cook for about 2-3 minutes.

Once the first side is golden, carefully flip each sandwich with a spatula. Cover again and cook for an additional 2-3 minutes.

Remove the sandwiches from the skillet and let them rest for a minute. Slice each sandwich diagonally and serve hot.