Instructions

• Drain the tuna and flake it into a large bowl.
• Add the finely diced celery, finely minced red onion, and finely minced flat-leaf parsley to the bowl; stir to combine.
• Add ½–¾ cup mayonnaise a little at a time (see notes), 1 teaspoon mustard (Dijon or stone-ground), and 1 tablespoon freshly squeezed lemon juice. Stir until the mixture is evenly combined.
• Season the tuna salad with salt and pepper to taste.
• Butter one side of each bread slice. Toast the buttered bread in the oven or in a toaster to your desired darkness, or leave the bread untoasted if you prefer.
• Spoon the desired amount of tuna salad onto each slice of toast.
• Place cheese slices on top of the tuna and melt the cheese—either briefly in the oven (or under a broiler) or in the microwave—until the cheese is melted to your liking.
• Thinly slice tomato and add the slices either under the cheese, on top of the melted cheese, or beside the tuna as you prefer. Optionally sprinkle the tomato with additional salt and pepper.
• Serve open-faced or place another slice of toast on top to make a sandwich.