This is best eaten right away, while the cheese is hot and melted. You can prepare the tuna fish mixture in advance and store it in the fridge. To make pickled red onions, slice half of a red onion and mix the slices with the juice of one lime, and let it sit in a bowl for an hour. You can use a slice or two of
that pickled red onion on top of this tuna melt to brighten things up and add some extra crunch, if you want!