Position rack in bottom third of oven; preheat to 400°F. Sprinkle main cavity of chicken with salt and pepper; fill with herb sprigs. Starting at neck end, slide fingers under skin of breast and upper part of legs, loosening skin. Spread 3 tablespoons herb butter under skin on breast and upper leg meat. Place chicken on rack in large roasting pan; tie legs together loosely to hold shape. Scatter onions around chicken. Brush chicken and onions with 2 tablespoons herb butter; sprinkle with salt and pepper.