Heat the oil in a frying pan and fry the onion and garlic over a medium heat for 4 minutes or until softened. Set aside to cool.
Place the beef, onion and garlic mixture, herbs, Parmesan, tomatoes and olives in a large bowl, season with salt and pepper and mix well. Divide the mixture into eighths and shape into 8 even-sized patties. Cover and chill for 30 minutes.
Brush the patties with a little oil and cook on a hot barbecue for 4—5 minutes on each side until slightly charred on the outside and medium in the centre.
Meanwhile, toast the halved focaccia quarters on the barbecue for 1 minute on each side until browned.
Spread the base of each bun with some basil mayonnaise and top with some of the rocket and a cooked patty. Tear the mozzarella ball into pieces and divide among the 8 burgers. Top with the lids and a basil leaf and secure with a cocktail stick. Serve with extra basil mayonnaise and rocket, if liked.