Spread the rabbit joints with 2 tablespoons of the Dijon mustard. Cover them and refrigerate for at least 4 hours. Heat half the duck fat or clarified butter in a flameproof casserole dish. Brown the rabbit pieces all over, then transfer them to a bowl. Deglaze the pan with the wine and add this to the rabbit.
Preheat the oven to 160°C/Fan 140°C. Heat the remaining fat in the pan and fry the onion, garlic and lardons or pancetta. When they have browned and softened a little, add the flour and cook for a minute or so. Then gradually add the chicken stock, stirring well after each addition to make sure there are no lumps.
Put the rabbit joints back in the casserole dish, cover and bring to a simmer. Put the dish in the oven and cook for 1¼ hours, until the rabbit is tender. (If using wild rabbit it will take longer than farmed.)
Remove the rabbit from the pan and keep it warm. Place the casserole dish on the hob and stir in the tarragon, crème fraiche and remaining mustard. Put the rabbit back in the dish and season with salt and pepper to taste. Serve with pilaf rice.