To make the barbecue sauce heat the oil in a pan and cook the onion and mushrooms for 3-4 minutes until softened. Add apple, 300ml (1/2pt) water, 75ml (5tbsp) Tomato Ketchup, 15ml(1tbsp) Worcestershire Sauce and 10ml(2tsp) English Mustard. Cook for 10-15 minutes until thickened.
Meanwhile make the burgers. In a bowl mix together the lean minced beef with 30ml (2tbsp) Tomato Ketchup, 10ml (2tsp) English Mustard and 10ml (2tsp) Worcestershire Sauce. Season and mix well.
Divide into 4 and shape into burgers. Cook on a preheated grill or barbecue for 4-6 minutes per side.
Serve in sesame burger buns with the barbecue relish, lettuce and corn on the cob.