In a food processor, whiz the garbanzo beans, lemon zest, lemon juice, cumin, half the cilantro, egg and some salt and pepper. Scrape into a bowl and mix with 3oz of the breadcrumbs and the diced onions.
Form 4 burgers, press remaining breadcrumbs onto both sides and chill for at least 10 minutes
Heat the oil in a large frying pan until hot. Fry the burgers for 4 mins each side, keeping the heat on medium so they don’t burn.
To serve, slice each bun and fill with a slice of tomato, a burger, a few red onion slices, some cucumber slices and the remaining cilantro.