Preheat oven to 350 degrees F (175 degrees C).
Finely chop 2 sprigs of the rosemary and mix with 2 tbsp of the butter and the garlic and lemon zest. Rub the butter on the outside of the chicken. Stuff the lemon inside the cavity. Season chicken with salt and pepper to taste, then scatter with the remaining rosemary leaves. Place chicken in a baking dish or roasting dish.
Roast in the preheated oven for 2 to 2 1/2 hours, or until chicken is cooked through and juices run clear. Cooking time will vary a bit depending on the size of the bird.