Put the beef and pork mince in a bowl with the onion, Worcestershire sauce, capers, salt and pepper. Mix together well, using your hands. Divide the mixture into quarters and shape into 4 even-sized patties. Chill for 30 minutes.
Brush the patties with oil and cook on a hot barbecue for 5—6 minutes on each side until lightly charred on the outside while just cooked through inside.
Meanwhile, toast the buns on the barbecue for 1 minute on each side.
Spread the insides of the buns with a little mustard, fill with the shredded lettuce, patties, tomato slices and dill pickles.