This is almost certainly an American thing, we just don’t have “tuna fish” in British parlance. A “tuna sandwich” would be understood to be a tuna mayonnaise sa

This is almost certainly an American thing, we just don’t have “tuna fish” in British parlance. A “tuna sandwich” would be understood to be a tuna mayonnaise sandwich, sometimes with surprise crunchy bits and sometimes without. You do occasionally see “tuna crunch” used to explicitly denote the addition of, most commonly, red onion and bell peppers.