Heat oil in a large frying pan over low heat. Add the onions and salted capers and cook very slowly for 15-20 minutes, stirring occasionally. When onions are soft and golden add the sage and balsamic vinegar. Cook onion over low heat for a further 5 minutes.
Mix together the pork mince, onion, garlic, sage and seasoning. Divide evenly and with your hands make 8 flat patties.
Heat a non-stick pan (BBQ/Grill) until hot. Cook for about 3-4 minutes each side or until cooked through.
Serve the burgers topped with the onion in a bun with fresh rocket and grated beetroot, and avocado.