Combine yogurt and garlic in a bowl and season with salt. Refrigerate until ready to serve.
In a food processor, combine chickpeas, 1/2 cup flour, cumin, egg and 1/2 tsp. salt and process until almost smooth. Add onion and pulse just to combine. Place remaining 2 Tbsp. flour on a plate. Form chickpea mixture into 12 1/2-inch-thick patties. Dredge each patty in flour.
Warm olive oil in a large skillet over medium heat. Add patties and cook until golden, turning once, about 8 minutes total (work in batches if skillet won't fit all patties). Slide patties into pitas, add tomatoes and lettuce, top with reserved yogurt sauce and serve.