Melt 2 tablespoons of butter in a large saucepan over medium high heat. Add the roast and sear on all sides until well browned; set aside.
In the same saucepan, melt the remaining butter, add the salt and onions and saute for 5 minutes. Return the meat to the saucepan and pour in the coffee and the mushrooms.
Bring to a boil, reduce heat to low and simmer for 5 hours, turning meat over halfway through cooking time.
To make gravy: Remove a cup of the coffee mixture from the saucepan, combine with the cornstarch, stirring until smooth, and return to the simmering pan. Mix well, remove from heat and serve.
