while not tuna salad, one of the best "fast food" tuna sandwiches in the Chicago area can be found at Portillo's locations: http://www.portillos.com/portillos/look/

Grilled Tuna Sandwich
$4.19
Grilled (not fried) yellow-fin tuna filet with lettuce, tomato, sliced red onion, and their homemade tartar sauce on a toasted bun

I avg three per month.