How to Make It
Preheat the oven to 450°. Melt 1 tablespoon of the butter in 1 tablespoon of the oil in a skillet. Add the leeks; cook over moderately high heat, turning, until browned in spots, 6 minutes. Transfer to a 9-by-13-inch baking dish, season with salt and pepper and roast for 10 minutes, or until beginning to soften.
Wipe out the skillet. Melt the remaining 1 tablespoon of butter in the remaining 1 tablespoon of oil. Season the chicken with salt and pepper. Add the chicken and rosemary to the skillet; cook over moderate heat until browned, about 10 minutes. Set the chicken on the leeks, skin side up, add the rosemary and roast for about 20 minutes, or until the chicken is cooked through and the leeks are very tender. Serve hot.