For the burnt-onion ketchup, heat a tablespoon of the oil in a large non-stick frying pan over a high heat. Add a small handful of the sliced onions and fry until dark golden-brown and almost charred.

Return to the heat if necessary, and cook down slowly to reduce to a ketchup consistency. Check the seasoning and add salt, pepper and vinegar as required. Put the ketchup in a clean, plastic tub ...