For the chicken: The night before you plan to cook the chicken, place it in a large nonreactive bowl and pour wine over. Marinate chicken in wine overnight in the refrigerator, turning at least once.
The next day, preheat the oven to 425 degrees F. Place garlic, rosemary, sage and salt and pepper on cutting board, and, using a sharp knife, chop them together as finely as possible.
Remove chicken from wine; reserve wine. Dry chicken with paper towels. Loosen chicken skin by running your fingers underneath it around the breast, thighs, legs and back, being careful not to tear the skin. Smear 2/3 of garlic and herb mixture between flesh and skin of breast, thighs, legs and back. Place the chicken on a vertical roaster in a roasting pan, brush it all over with olive oil and rub remaining garlic and herb mixture over the skin.
Place chicken in oven and immediately reduce temperature to 350 degrees F. Roast 30 minutes. Remove chicken from oven, give it a º turn clockwise and brush it with juices that have accumulated in the roasting pan. Cover top section of chicken with a piece of foil just to the wings to keep it from getting too brown. Roast 1 hour, turning and basting the exposed part of the chicken after 30 minutes.
After 1 hour, test chicken for doneness by inserting the tip of a sharp knife in to the thickest part of the thigh. If chicken is done, the juices that run out will be clear. If they are not, return chicken to oven for 10 minutes. When done, transfer it to a cutting board; let rest 10 minutes.
For the sauce: While chicken roasts, pour wine from soaking chicken into a nonreactive saucepan. Bring to a boil over high heat and cook until reduced by 2/3. Set aside.
While chicken is resting, pour juices from roasting pan into defatting cup (or skim fat from surface of juices and set aside).
Transfer 2 tablespoons of fat to small, nonstick skillet set over medium heat. Add onion and cook 3 minutes. Add reduced wine and cook 1 minute. Add defatted pan juices, discarding remaining fat, and ? cup chicken stock and bring to a boil. In a small bowl, combine cornstarch with remaining º cup chicken stock and stir into boiling sauce. Boil, stirring, until sauce thickens, about 1 minute. Reduce heat so sauce just simmers, taste, and season with additional salt, if necessary. Stir in sage.
Carefully pour any juices that have accumulated on the serving platter into sauce. Spoon some of sauce over chicken and transfer the rest to a gravy boat. Serve chicken immediately and pass gravy boat.
