Turn your oven to broil on high.
In a large bowl, whisk together the vinegar, oil, mustard, salt, and pepper. Add the tuna, pickles, herbs, and bell pepper and toss to coat.
Scoop the tuna onto your toasted bread and top with 1-2 tomato slices (if using) and then a slice (or two) of cheese.
Place them on a baking sheet (don’t use parchment) and broil for about 2 minutes (keep an eye on it!), or until the cheese is melted and bubbling.
I believe tuna melts are best eaten with a knife and fork, but you could put a piece of toast on top to make it a sandwich. Enjoy!