I have both low expectations and high standards for tuna salad sandwiches, which means that I'm always excited to find one that's worth paying for. (Anyone with a jar of Hellmann's and a lemon can make great tuna salad, and the sky's the limit as far as other add-ins.) Same goes for tabbouleh: Provided you've got a sharp knife and some patience, tabbouleh is only as tricky to make as mincing a bunch of produce and eating it as soon as possible—before the parsley and mint get completely saturated by the tomato and lemon juices.
Impressively, the couple behind Kookoo Cafe in Brookline Village nail both components in the Terrifying Tuna ($7.25). Leaving aside the sandwich's weird name—several of the cafe's specialties have equally goofy labels—it's a winning package of lightly dressed-almost fluffy-tuna salad and tabbouleh that's loaded with finely minced parsley and lemon juice, sweetened with diced tomatoes, and lightly bound with bulgur. This more traditional version of the classic Mediterranean salad (light on the grain, heavy on the herbs) is so refreshing, I usually ask them to pack in a triple dose with the tuna.
Cucumber half-moons, mesclun greens, and a smear of thick honey mustard fill out the rest of the sandwich, which happens to be tightly wrapped in a round of excellent pita bread from a bakery in Worcester. And because I have yet to find a sandwich that they don't improve, I often add slices of buttery avocado.