Highly sought after by certain ladies, this delicacy is another term for the spitroast, where two gentlemen entertain one lady at the same time. Usually orally and vaginally, but other configurations are possible. The lady is obviously the "Tuna" in the aforementioned sandwich.
"My twin brother rang me and told me that his new girlfriend was hot for a threesome, but I wasn't sure until she came on the line and insisted on it. So I ended up serving up a hot tuna sandwich that afternoon."