How to Make It
In a large saucepan, dissolve the sugar in the water. Cook over moderate heat without stirring until a medium-amber caramel forms, about 5 minutes. Off the heat, carefully add the butter and 1/2 cup of the balsamic vinegar. Return the saucepan to the heat and cook until the caramel is melted.
Add the onions, lemon zest, bay leaves and chicken stock to the saucepan and bring to a boil. Season with salt and pepper and simmer over moderately low heat until the onions are very tender and glazed and the liquid is syrupy, about 1 1/2 hours. Stir in the remaining 2 tablespoons of balsamic vinegar and season with salt and pepper.