Prepare a high gas or charcoal grill fire.

Heat the oil, garlic, and cumin in a small saucepan over medium-low heat until the garlic is soft but not brown, about 3 minutes. Set aside off the heat.

Tear one of the pitas in half and toast in a toaster until golden-brown and crisp. In a food processor, grind the pita into fine crumbs; transfer the crumbs to a small bowl and measure them—you’ll need 1/2 cup. If necessary, toast and grind a second pita.

In the food processor, purée 1 can of the chickpeas with the oil mixture, 2 Tbs. of the tahini, 1 Tbs. of the lemon juice, the egg, and 3/4 tsp. salt until smooth. Add the remaining chickpeas and the pita crumbs, parsley, and cilantro; pulse until the chickpeas are coarsely chopped and the mixture is well blended.

Put the remaining 3 Tbs. tahini in a small bowl and gradually whisk in 2 Tbs. water and the remaining 3 Tbs. lemon juice (the tahini will thicken at first and then smooth out). Shape the chickpea mixture into 6 patties, each about 3/4 inch thick. Brush both sides of each with oil. Grill, covered, until heated through and nicely marked on both sides, 2 to 3 minutes per side.

Cut the remaining 3 pitas in half and heat briefly on the grill. Layer the burgers in the pitas with the sliced cucumber and tomatoes, drizzle with the tahini sauce and harissa (if using), and serve.

nutrition information (per serving):
Calories (kcal): 540; Fat (g): 23; Fat Calories (kcal): 210; Saturated Fat (g): 3; Protein (g): 20; Monounsaturated Fat (g): 12; Carbohydrates (g): 67; Polyunsaturated Fat (g): 6; Sodium (mg): 660; Cholesterol (mg): 30; Fiber (g): 13.