Combine chickpeas, garlic, onion and spices in a food processor; pulse until well mixed. Add flour and baking powder; pulse until well combined. Add enough breadcrumbs until it all holds together in a thick paste, much like a homemade burger mixture would. Form the mixture into medium-sized balls, and flatten into patties. Fry in a small amount of oil until golden brown on both sides (5-7 minutes). Serve hot on a burger bun with celery leaves or lettuce and tzatziki sauce. Yields 4 patties.