How to Make It
Melt the butter in a large saucepan. Add the onion and thyme and cook over moderate heat, stirring occasionally, until the onion is softened and browned, about 10 minutes. Add 1/2 cup of the wine and boil over moderately high heat until the liquid is almost evaporated, about 3 minutes. Add the chicken stock, bay leaf and peppercorns and boil over moderately high heat until reduced to 1 1/2 cups, about 30 minutes.
In a medium saucepan, combine the balsamic vinegar with the shallots and the remaining 1/2 cup of white wine and boil over moderately high heat until the liquid is reduced to 1/4 cup, about 8 minutes. Add the reduced chicken stock and cook for 2 minutes. Strain the sauce into a small saucepan.
In a bowl, combine the butter with the thyme, tarragon, parsley and garlic. Gently spread the mixture under the chicken skin. Season with salt and pepper and refrigerate until the butter is firm, about 1 hour.
Preheat the oven to 450°. Lightly oil a large, heavy roasting pan and set the pan over 2 burners. Add the chicken, skin side down, and cook over moderately high heat until browned, about 5 minutes. Turn the chicken skin side up and roast in the oven until cooked through, about 20 minutes for the breasts and 25 minutes for the thighs and drumsticks; transfer the chicken to a warm platter.
Pour the juices from the roasting pan into the balsamic jus. Reheat the balsamic jus, then pour into a sauceboat and serve with the chicken.