Place an oven rack in the lowest part of oven. Preheat oven to 475 degrees F. Season chicken with salt and pepper and set skin-side down on a non-stick baking sheet. Roast 10 minutes. Remove the chicken from the oven, turn the pieces over, place back in the oven and roast 10 minutes more.
Set the chicken, skin-side up, in a 9- x 13-inch casserole.
Lower oven temperature to 350 degrees F. Heat the oil in a skillet set over medium to medium-high heat. Add leeks and mushrooms and cook until tender, about 5 minutes. Mix in garlic, flour and thyme and cook 2 minutes more. Stirring constantly, slowly mix in the stock and wine. Bring to a simmer and season with salt and pepper. Pour the mixture over the chicken. Cover and bake 60 minutes, or until chicken is tender. Sprinkle with chopped parsley, if desired, and serve.