I've been vegetarian for several years, but am craving a tuna melt!?

It's probably not due to a diet that's lacking, but moreso a specific feeling you got from eating a food you used to love. From my own personal experiences, if you ate the tuna melt it wouldn't taste as good as it used to. In fact if you've been a strict vegetarian for years, it will probably taste downright disgusting to you. I too would suggest trying a bit of it. If I think I'm craving meat and I have the tiniest taste all cravings quickly go away because it just doesn't taste like it used to. Your tastebuds change a lot more quickly than your mind does. :)