Legend has it that the tuna melt was accidentally invented in the 1960s at the Woolworth’s lunch counter in Charleston, S.C., when the cook didn’t notice that a bowl of tuna salad had tipped over onto a grilled cheese. We may never know if this story is true, but there’s no doubt that the tuna melt has become a classic American diner food. This recipe adds chopped cornichons and whole-grain mustard for a satisfying crunch and vinegary element. Extra-sharp Cheddar is a must, and as with…