Heat the oil in a non-stick frying pan. Sauté the onion and garlic for approx. 3 mins., transfer to a bowl, allow to cool slightly. Add the meat, breadcrumbs and parsley. Shape into a compact mass by hand, divide into 4 portions, shape into 4 burgers.
Cover and grill the burgers over/on a medium heat (approx. 200 °C for approx. 5 mins. on each side. Grill the bacon alongside for the final 2 mins. Place the cheese slices on top of the burgers. Grill the halved burger buns alongside for approx. 1 min. Spread mayonnaise on the bottom half of each bun, spread ketchup on the top half. Add the burger, bacon, lettuce, tomatoes and gherkins, cover with the top half of the bun.