Place roast in a roasting pan that has a tight fitting cover.
Combine the remaining ingredients in a bowl and mix well. Then pour over the roast. Liquid should be a minimum of 1/4 way up the roast.
Place roast in a preheated oven at a temperature of 325F. (Cooking time is 25 mins/pound) Cook for 3 hours and an internal temperature of 130F.
Transfer roast to a plater and cover loosely with tinfoil and let rest. For the gravy, Pour all the liquid, mushrooms and onions into a pot and bring to a rolling boil. Add cornstarch according to its directions to achieve desired thickness.
