Top tuna with cheese, packing it on generously, and slide sandwiches under broiler. Broil until cheese melts and bubbles, 4-5 minutes. Remove tuna melts from broiler and serve immediately.
However, I've already found the best club sandwiches and best roast-beef subs to order, so in the name of research, I got tuna at Jimmy John's, Jersey Mike's, and Subway to see which popular chain ...