Prepare a bowl that you will use to mix all the ingredients.
Now you have to drain the tuna and place the drained tuna into the bowl.
Add the celery, onion, pickles and jalapeño peppers finely chopped with the tuna. You can chop them with a knife or using your kitchen robot.
Once you have all the dry ingredients in the bowl you can mix them using a fork.
Add the mayonnaise and dijon mustard. You can start with one tablespoon each and add more depending on your taste.
Spread the butter on the external side of the sandwich bread and stuff your sandwiches with the tuna mixture.
Place one cheese slice (or two) on top of the tuna filling and put the other slice of bread to prepare your sandwich.
Grill your tuna melts using a pan, or if you have an oven try is even easier to grill your tuna melts preserving the cheesy filling into your oven.
Cut each tuna melt and enjoy.