In a food processor, whizz the chickpeas, lemon zest, lemon juice, cumin, half the coriander, the egg and some seasoning. Scrape into a bowl and mix with 80g of the breadcrumbs and the diced onions. Form 4 burgers, press remaining breadcrumbs onto both sides and chill for at least 10 mins.
Heat the oil in a 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, a dollop of chilli sauce and the remaining coriander.