Kosher salt and freshly ground black pepper (to taste)
1cupshredded cheddar cheese(or cheese of your choice)
Preheat a toaster oven or the regular oven to 375 F. Toast the English muffin halves for about 3 minutes until very lightly browned. Remove and leave the oven on.
Mix the tuna, mayo, onion, celery, relish, mayo, and mustard in a medium-sized bowl. Add the salt and pepper and blend well.
Divide the tuna over the 4 muffin halves. Top each with 1/4 of the shredded cheese and return to the oven for about 5 minutes until the cheese has melted. Serve warm. If you want to give the tuna melts a minute under the broiler at the end to really make sure the cheese is melty, go for it.
Solid white tuna results in a salad that can be too dry unless you add an unconscionable amount of mayonnaise, and using all light tuna can be perceived by kids (and adults) as too fishy. The 50/50 combo results in a tuna fish that is nicely in the middle.
You can use any kind of mustard here, but I like Dijon for its strong spiciness.
Make the tuna up to 3 days ahead of time, and assemble and bake the sandwiches whenever you want one.
You can use chopped pickles, such as cornichons or half sours, instead of the relish.