Trim the steak and cut into neat pieces for skewering – try to allow about four pieces of meat per person. Peel the onions and cut in half, then separate out into pieces. Skewer pieces of meat, pushing them close together with onion at either end if they are to be cooked ‘rare’ or separate the pieces of meat and skewer alternating with pieces of onion and mushrooms for meat that’s to be well done. Brush with oil and set aside while preparing the sauce.
Heat the oil in a saucepan and add the onion. Fry gently for a few minutes to soften the onion, then stir in the tomato ketchup, water, vinegar, sugar, mustard, Worcestershire sauce and salt. Bring up to the boil and simmer for 5 minutes. Blend a little of the hot sauce with the cornflour blend, mix well and return to the saucepan. Stir until boiling and slightly thickened.
Meanwhile, preheat the grill until hot. Set the kebabs under the grill at least