Tip the beans into a bowl and mash roughly with a potato masher, leaving about a third whole for texture.
Add the onion, garlic, spices, egg and half the breadcrumbs, then mix well. The mix should hold together when squeezed.
Shape into 4 large patties with floured hands and coat each in the remaining breadcrumbs.
Chill the patties on a plate for 10 minutes to firm up.
Heat the oil in a frying pan over a medium heat and cook the burgers for 3-4 minutes on each side until crisp and heated through.
Serve in the buns with lettuce and ketchup.