Remove the chicken from refrigeration 30 minutes prior to cooking. Preheat the oven to 220°C (450°F) for a standard oven or 200°C (430°F) for convection. Position the oven rack in the center. Combine all ingredients for the butter mixture, incorporating the juice from two lemon wedges.
Position the chicken in a roasting pan. Gently separate the skin from the meat of the breast and drumsticks using the back of a spoon. While the bird is propped upright, distribute most of the butter mixture underneath the skin, reserving a portion for the surface.
Coat the chicken’s exterior with the remaining butter and squeeze the juice of two additional lemon wedges over the surface. Insert the used lemon wedges and rosemary sprigs into the cavity. Tie the drumsticks with kitchen twine and tuck the wings under the body. Season the chicken generously with salt and pepper.
Arrange the onion and garlic in the roasting pan and place the chicken on top. Pour the wine or broth around the chicken and drizzle the bird with olive oil.
Place in the oven and roast for 10 minutes. Then reduce the temperature to 180°C (350°F) and continue roasting for approximately 1 hour and 15 minutes, or until the internal temperature reaches 75°C (165°F) and juices run clear when pierced between the thigh and body. Baste twice during roasting—once at 30 minutes and again at the 1-hour mark—with pan juices.
Once cooked, allow the chicken to rest for 15 minutes without covering to maintain skin crispness. Serve carved with pan juices. The garlic may be used, but the onion is typically discarded. Refer to notes for additional serving suggestions and broth preparation using the carcass.