Toss the onion in olive oil and salt. Slowly caramelize onions in a large saute pan over medium heat until a deep brown in color. Deglaze the pan when necessary with a splash of white wine. When the onions are caramelized, remove onions from pan. Add reduced balsamic vinegar to the onions and puree together in a blender. If onions will not blend, add an additional 1 tablespoon of balsamic vinegar of the mixture and puree.