In a large skillet, heat a drizzle of oil over medium heat; add the onions and cook, stirring often, for about 10 minutes or until golden and caramelized. Add the garlic and balsamic vinegar and cook for another 2 minutes, until deep golden. Set aside to cool slightly.
Transfer to a medium bowl and stir in the sour cream, salt and pepper. If you want it more finely chopped or smooth, pulse it in the food processor. Cover and chill for at least an hour before serving.
Makes about 1 1/2 cups.