* To flatten the chicken: Stand the chicken upright and cut out the backbone with a large kitchen knife. Spread the chicken open on a board with the skin side down. Cut around and remove the breast bone with a boning knife. (Or you can ask the butcher to do all of this for you.)
To cook the chicken: Sprinkle the chicken with 1 teaspoon salt on each side.
Combine the olive oil, lemon zest, lemon juice, garlic, rosemary and 1 teaspoon pepper in a ceramic or glass dish just large enough to hold the flattened chicken. Add the chicken, and turn to coat. Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight, turning once or twice.
When ready to grill, prepare a hot charcoal fire on one side of grill (or turn a gas grill on low heat). Spread 1/4 of the coals across the other side of the grill.
Place the chicken on the cooler side of grill skin side up, and weight it down with the dish you used for marinating. Cook for 12 to 15 minutes, until the underside is golden brown.
Turn the chicken skin side down, weight again with the dish, and cook for another 12 to 15 minutes, until the skin is golden brown and the chicken is cooked through.