Sautee onions in butter until soft, about 10 minutes. Add garlic and cook about 1 minute, then add brown sugar and stir using a wooden spoon. Add vinegar, ketchup, lemon juice, Worcestershire sauce and salt. Bring to a simmer.  Preheat oven to 375F. Place chicken breast in a baking dish and pour sauce over the chicken. Bake until internal temperature of 165F is reached.