Cut the buns in half. In a small bowl, combine 1/4 cup mayonnaise, whole onion, mustard, Worcestershire sauce, and tuna (if tuna is canned in large chunks, mash it first with a fork).
Grease each of the six halves of the buns with tuna and put one tomato ring on top. Put these pieces on a baking sheet.
Turn on the oven to preheat to 200 degrees. In a separate bowl, mix together the cheese and the rest of the mayonnaise, and spread over the tomato slices. Put the baking sheet in a preheated oven, bake the tuna sandwiches for 11-13 minutes, until the cheese melts.