How to Make It
Preheat the oven to 350°. In a large enameled cast-iron casserole, melt the butter in the oil over moderately high heat. Season the chicken inside and out with salt and pepper. Add the rosemary to the cavity and tie the legs together.
Set the chicken in the casserole on 1 breast and cook until browned, about 4 minutes. Carefully turn the chicken onto its other breast and cook until browned, about 4 minutes longer. Turn the bird on its back, scatter all the vegetables around it and season with salt and pepper. Cover and roast in the oven for about 1 1/4 hours, or until the juices run clear when a thigh is pierced; stir the vegetables every 20 minutes or so. Transfer the chicken and vegetables to a platter, cover and keep warm.
Skim the fat from the juices in the casserole. Boil, stirring, until reduced to 1/2 cup. Season with salt and pepper. Pour the sauce over the chicken and serve.