Melt half the butter and cook the sliced mushrooms gently for ten minutes. Season lightly and cool. Stuff the pheasant with three-quarters of the mushrooms. Brown the bird in the rest of the butter, and put in a casserole. Blend the flour into the pan juices, and gradually work in the stock and sherry. Season well and pour over pheasant. Cover tightly and cook in the oven at 325°F, 170°C, gas mark 3 for one and a half hours, but allow an extra thirty minutes for an old bird. Just before serving, add remaining mushrooms, and garnish with watercress. The sauce is rich and delicious.