Rinse the cavity of the chicken. Squeeze some of the lemon juice into the chicken cavity and season well. Stuff with rosemary, onion and garlic and squeezed lemon wedges. Place bird on the rack of a roasting pan. Squeeze remaining lemon juice over chicken; rub with a little olive oil and season well. Place in a preheated 200°C oven and roast for about 1 ½ hours. You need only baste the chicken once or twice during roasting, paying particular attention to the breast area, which is the leanest and therefore dries out the quickest. Remove from oven, cover with foil and allow to rest for at least 15 minutes before serving.