Heat the oil in a deep frying pan or flameproof casserole with a lid. Add the chicken pieces and brown on all sides.
Reduce the heat and add the onions. Cook for a few minutes until beginning to soften.
Add the potatoes to the pan, then stir in the crème fraîche followed by the stock, mustard and thyme.
Cover and cook for 40 minutes on a low heat until the potatoes are tender.
Mix the cornflour with an equal amount of cold water to form a smooth paste, then stir into the casserole. Cook stirring until the sauce thickens slightly. Serve with a scattering of fresh thyme leaves if desired.