Fry the rabbit and onion in 2 teaspoons oil in a flameproof casserole until golden brown. Trim the fennel, and slice into ¼ in / 0.5 cm pieces. Put it with the rabbit and onion. Cover with stock, water or wine, and bring to the boil. Season very lightly, and turn the heat down to the merest simmer. Cover and cook gently for about 45 minutes. The dish can be prepared to this point the day before, in which case strain and reduce the cooking juices to about ¼ pt / 140 ml, and then cool as quickly as possible before refrigerating until required, keeping the rabbit and vegetables separate from the stock. Remove the rabbit from the bone, if preferred. Fry the mushrooms briskly in the rest of the oil until they give off most of their moisture. Lightly oil or butter a pie dish, and lay the pieces of rabbit in the bottom, then a layer of mushrooms, a layer of fennel and onion and finally the remaining mushrooms. Tuck a few tarragon leaves into the filling, and sprinkle lemon zest on top. Add the prunes, if using. Spoon the, by now, jellied stock over the filling. Roll out the pastry and use to cover the dish, pressing well down to seal it. Make a hole in the top for steam to escape, and decorate with pastry leaves or other trimmings. Brush with milk or cream, to glaze, if you wish, and bake in a preheated oven at 190°C / 375°F / Mark 5 for 35 minutes. Serve hot or warm with green vegetables or a crisp salad.