Combine the minced rosemary, salt, and pepper in a small bowl. Gently loosen the skin from the chicken and spread half of the rosemary mixture under the skin. Coat the outside of the chicken halves with 1 tablespoon of olive oil each, then rub with the remaining rosemary mixture. Place each chicken half in a resealable plastic bag and add 1/4 cup of wine to each bag. Seal the bags tightly and refrigerate for 2 hours.

Reserving the marinade, remove the chicken from the bags. Pat the chicken dry with paper towels, and sprinkle generously with salt and pepper. Preheat oven to 375˚F.

In a wide, deep, oven-proof frying pan, heat 2 tablespoons of olive oil over medium heat. Place the chicken halves breast-side down in the pan (one at a time if neccesary) and brown for about 5 minutes. Transfer to a dish and set aside.

In the same pan, sauté the onions for about 20 minutes, stirring occasionally, until softened and golden. Reduce heat to medium-low, add the last tablespoon of olive oil, and stir in the minced garlic. Pour the reserved marinade, the last 1/4 cup of wine, and the chicken stock into the pan. Scrape up any browned bits from the bottom of the pan. Transfer the chicken halves back into the pan, cover, and place in the oven. Roast about an hour, basting occasionally, until a thermometer inserted into the thickest part of the thigh, away from the bone, reads 170˚F. Remove chicken from oven and place on a cutting board. Loosely cover with aluminum foil and allow to rest for 20 minutes.

Return the pan to the stovetop over medium heat. Add the sprig of rosemary and cook 3-5 minutes, until aromatic. Discard the rosemary. Season the sauce to taste with salt and pepper. Carve the chicken, and serve with the sauce.