Fry the chicken thighs in a deep frying pan or casserole with a lid in a little oil to brown the skin. Turn over, add the shallots and brown them for a minute. Add the potatoes, cream and stock and bring to a simmer. Season. Add the mustard and half of the tarragon, then simmer with a lid on for 40 minutes, or until the potatoes are tender and the chicken cooked, turning them over if needed. Scatter with the rest of tarragon.