In a bowl, combine tuna with celery, mayo, lemon juice, olive oil, salt, and pepper.
Using a fork, lightly mash the tuna mixture, but don’t over-mash. You want to keep some of the tuna chunks intact.
Toast the bread in a pan with a little bit of water or oil until nice and toasty.
Make the sandwiches by spreading mayo over the bread, then top with tuna salad, slices of tomato, and slices of cheese.
If the cheese needs help to melt, place on a sheet pan and under the broiler for 2-3 minutes or until melted.
Close the sandwiches with bread, cut diagonally into triangles, and serve while still warm.
Enjoy your Tuna Melt!