1. Preheat the oven to 190°C.
2. In a small bowl, mix together the garlic, black pepper and oil. Gradually beat in the parsley, hazelnuts and softened butter. Set aside.
3. Gently slide your hand under the skin of the chicken to loosen it. See if you can get into the leg part as well but being careful not to split the skin. Rub the hazelnut butter under the skin.
4. Place onion slices in the bottom of a roasting dish and sit chicken on top. Stuff the lemon slices into its cavity. Secure cavity with a skewer or toothpick and pull the chicken legs together and tie them together with some cooking twine. Squeeze over lemon juice and rub over a little olive oil.
5. Place in the oven to roast for 1 hour and 15 minutes, basting once or twice by spooning the juices over the chicken. You can also test if chicken is cooked by skewering the thickest part of the leg – the juices should run clear.