1. Preheat the oven to 180°C, gas mark 4. Make the stuffing: heat the oil in a small frying pan and add the onion. Cook for 3-4 minutes, until softened, then add the garlic and cook for a further minute. Remove from the heat, tip into a bowl and allow to cool. Remove the stalks from the parsley (reserve for making the stock later) and chop the leaves.
2. Stir in the breadcrumbs, parsley, and lemon zest and juice, and season. Stir in enough of the beaten egg to bind the mixture together.
3. Push the stuffing into the smaller neck cavity of the bird and smooth the flap of skin back over it. Secure with cocktail sticks if necessary. Sit the chicken in a roasting tin and tie the legs together with string. Smother the breast and legs with the softened butter and season well. Roast for 1 hour 30 to 1 hour 45 minutes, basting the chicken a couple of times. Test to make sure the chicken is cooked by piercing the thickest part of the thigh with a skewer. If the juices run clear and there is no pink meat, the chicken is cooked. If not, continue cooking for 15 minutes and test again. Transfer the cooked chicken to a board and cover with foil. Rest for 15 minutes before carving.