Heat the oil and butter in a large flameproof casserole dish over a moderate heat. Add the chicken and brown it all over for 5 minutes, then remove to a plate.
Add the onions and carrots to the casserole dish and cook gently for 5 minutes until lightly coloured.
Pour the stock and wine into the dish, then add the thyme, orange zest, nutmeg and salt and pepper. Bring to the boil, then reduce the heat and return the chicken to the dish. Cover and simmer gently for 1 hour, or until cooked through, basting the chicken with the juices from time to time. Lift out the chicken and carve into serving pieces. Cover and keep warm.
Using tongs, remove the cooked thyme from the casserole. Stir the crème fraîche or cream into the cooking juices and simmer gently, uncovered, for several minutes to thicken slightly. Put the chicken pieces on top of the cooking juices and serve from the dish or arrange on a serving dish. Garnish with sprigs of fresh thyme.