How to Make It
Preheat the oven to 475°. In a large enameled cast-iron casserole, heat the oil over moderately high heat. Season the chicken inside and out with salt and pepper and tie the legs together. Set the chicken in the casserole on 1 breast, cover and roast in the oven for 30 minutes. Carefully turn the chicken onto its other breast, cover and roast for 20 minutes. Turn the bird on its back, cover and roast for 20 minutes longer, or until the juices run clear when a thigh is pierced. Transfer the chicken to a platter, cover, and keep warm.
Skim the fat from the juices in the casserole. Add the shallot and cook over low heat until soft, about 4 minutes. Add the vermouth and prosciutto and boil, stirring, until the sauce is reduced by half, about 4 minutes. Pour in any collected chicken juices. Add the peas and simmer just to heat through. Remove from the heat and swirl in the butter until blended. Season the sauce with pepper, pour it into a sauceboat and serve with the chicken.