In a medium bowl, mix the tuna, onion, scallion, parsley, mayonnaise, and a few drops of hot sauce. Season with salt and pepper.
Butter 1 side of each slice of bread and turn them over. Pile the tuna mixture onto the non-buttered sides of 2 slices of bread and top with the Cheddar. Sandwich with the other 2 slices of bread, buttered side up.
In a dry skillet over medium heat, cook the sandwiches until the bread is crunchy and the cheese has melted, 2 to 4 minutes per side.
Cut the tuna melt in half and, if desired, set out some ketchup on the side.