Heat the oil in a deep flameproof casserole dish over a high heat. Add the chicken thighs and fry for 7-8 minutes, turning once until golden brown. Remove from the dish and set aside.
Add the mushrooms and leeks to the casserole and cook over a high heat for 3-4 minutes, stirring, until beginning to soften. Stir in the stock and bring to the boil. Return the chicken to the dish and simmer, uncovered, for 10 minutes until the liquid has reduced by about one-third. Preheat the oven to 220°C/gas mark 7.
Stir the mustard and crème fraîche into the casserole and simmer for 10 minutes until the sauce has thickened slightly. Stir in the chopped tarragon and season to taste.
Arrange the crispy sliced potatoes on top of the chicken and vegetables, covering them completely. Bake in the oven for 20 minutes until the potato topping is golden and crispy.