Preheat the oven gas 6, 200°C, fan 180°C. Rub the butter all over the chicken, season well and tuck the halved onion inside the cavity. Sit the chicken in a casserole dish with a lid (if you don't have one, a snug roasting tray with foil will do). Tuck the rest of the garlic, bay and thyme around and pour in the water and wine. Roast for 30 minutes until lightly golden, place the lid on top (or cover with foil) and continue cooking for another 50 minutes to 1 hour until the chicken is cooked through. Test by inserting a skewer in the thickest part of the thigh and checking that the juices run clear. If not, cook for a further 10 minutes. Leave to rest, covered, for 10 minutes. Carve and serve with a potato salad, steamed green beans and crusty bread to mop up all the juices.
