2To make the dressing, put the tahini, lemon juice, garlic, and a pinch of salt into an airtight jar; close the jar and shake vigorously. Add the olive oil and shake again. Thin with water until pourable, if necessary. Set aside.
3In a bowl, toss together the cucumber, tomato, red onion, and parsley. Season with the sumac and a little salt.
4To make the wraps, toast the lavash in a large frying pan. For each wrap, layer hummus, the cucumber mixture, and 3 pieces falafel. Drizzle with some of the harissa and tahini dressing. Roll up burrito-style and serve.