Preheat oven to 425°F. Spray a large 2 inch deep baking pan with cooking spray.
Brush chicken inside and out with butter. Season with salt and pepper.
Stuff chicken with lemon, 1/2 onion, and 2 sprigs rosemary. Tie legs with cooking twine and tuck wings under.
Arrange onions in pan. Center chicken on top. Rub with seasoning.
Place into the oven, baking for one hour. Check top for browning and lay foil loosely on top, if needed. Bake for an additional 30 minutes or until juices run clear from thigh and leg portions.
Rest tented with foil for 20 minutes then carve and serve.