Mix together the lemon zest and juice, garlic, onion, olive oil, salt and pepper. Place the chicken and lemon mixture in a resealable plastic bag and seal well. Leave the chicken to marinate in the refrigerator overnight.
Preheat the oven to 350° F.
Take the chicken out of the marinade and place in a roasting pan. Roast until cooked through, 30–45 minutes, depending on the size of the chicken legs. When the chicken is pierced with a knife, the juices should run clear. Sprinkle with chopped parsley.