My go-to can-of-tuna meal is two 5-oz cans (or one 12-oz can), a heavy dollop of mayo, and a spoonful of sweet relish. Stir it all up, serve in a bowl.
This used to be my standard recipe for tuna fish sandwiches, but then I figured, why add bread? That’s just extra calories. I’d rather have a bit more mayo than two slices of dry bread.
EDIT: I prefer two 5-oz cans because I squeeze out all the water from the cans (open with can opener, use lid to compress and squeeze out water) and it’s harder to squeeze out most of the water from the large 12-oz can. Two smaller cans works better. I don’t like my tuna watery. My wife does, though; she says it adds a stronger tuna-y flavor to the dish.