Cut large oxtail into 4-inch segments and blanch in cold water for at least 6 hours. Drain well and dry lightly with a clean towel.
Strain sauce into a bowl through a fine sieve. Place pieces of oxtail in a clean casserole and garnish with mushrooms which you have sautéed lightly in
Skim fat from surface of sauce; pour sauce over the meat and vegetables. Bring slowly to a boil; cover the casserole and cook in a slow oven (325°-M2) for 1 hour. When the hochepot is done, the meat should be meltingly tender and the sauce rich and smooth, and slightly thick without the aid of flour or a beurre manié.
Sprinkle with finely chopped parsley and serve from the casserole with a purée of peas or chestnuts.