Form ground beef into two patties.
Heat olive oil in a medium pan. Add the onions and cook on medium heat, stirring constantly for 5 minutes until caramelized. Add balsamic and continue to cook until evaporated. Remove from heat and set aside.
Cut parsnips into strips
Preheat oven to 450 degrees. Toss parsnips with olive oil and spread out on a large rimmed baking sheet. Roast for 10 minutes; turn parsnips and roast until parsnips are tender and browned in spots, 10–15 minutes longer.
Meanwhile heat the same pan you cooked the onions in with a drizzle of oil. Sprinkle the meat with salt and pepper, and gently mix it by hand without breaking up the meat too much. Form it loosely into patties. Try not to compress the meat too much.
Place the patties on the pan – it should be hot enough that they sizzle. At no point should you squash the burger – keep the juices inside! Cook until the first side is well browned, then flip. Remove the burgers from the pan when they are done to your liking.
Place two lettuce leaves on a plate, then top with burger patties, caramelized onion and your favorite burger accompaniments. Top with another lettuce leaf and serve with parsnip fries.