They had a similar item on a Coco's Restaurant menu when I worked there. It was called "Have -a-Heart" and I loved it then, too. One substitute for the Greek yoghurt is Lebni. It has even less water than the yoghurt and is just a tad less sour. Also, when cutting the tomato, I did a simple VanDyke cut (large zig-zag) that makes a better vessel for the tuna. Love the capers in the tuna!
Love this cheesy tuna melt! It's quick and easy and makes for a great for a weeknight dinner.