In a bowl, mix together the ketchup, mustard, treacle, vinegar and oil until smooth.
Thread the turkey meat on to 8 skewers alternately with the chunks of pepper and onion and lay the kebabs side by side in a shallow dish. Spoon or brush the marinade over the turkey, turning the kebabs over so both sides of the meat are coated.
Cover the dish with cling film and leave in a cool place for 3-4 hours or until ready to cook.
Barbecue the kebabs for 6-8 minutes or until the turkey is cooked and the vegetables scorched at the edges, brushing any remaining marinade over the turkey as they cook.
Serve hot with relishes and a selection of salads – a potato salad goes particularly well with the kebabs.
Tip: If using wooden skewers, soak them for 30 minutes in cold water first to prevent them catching fire on the barbecue. Although the kebabs won’t be cooked for several hours, covering the dish with cling film will prevent the skewers drying out.