Prepare outdoor grill with hot coals arranged for indirect grilling, or heat gas grill for indirect grilling.
In food processor, process butter, garlic, thyme, rosemary, sage, salt and pepper until well-blended. Scrape into small bowl. Grate the rind from lemon; add rind to herb butter.
Cut lemon in half. Place the lemon halves, the onion, sprig of thyme and rosemary in cavity of chicken. With fingers, gently separate skin from meat of breasts, thighs and legs. Spread herb butter in between the skin and meat. Tie legs together and bend back wings.
Grill chicken over indirect heat, covered, 1-1/4 hours or until instant-read thermometer inserted in thickest part of thigh without touching bone registers 180°F. Let stand 15 minutes before carving. Makes 4 servings.
Heat oven to 375°F. Place chicken on rack in roasting pan. Roast in 375°F oven for 1-1/4 hours or until internal temperature registers 180°F.