Put the brisket in a roasting pan and scatter the mushrooms, carrots, celery and onions on top. Whisk together the ketchup, brown sugar, soy sauce, vinegar, Worcestershire and 2 cups water in a bowl and pour it over the top. Cover the pan with foil and cook for about 5 hours.
Transfer the brisket to a plate, cover it with foil and let it rest for 20 minutes. Transfer the vegetables to a plate and cover with foil to keep warm. Skim most of the fat from the liquid and strain.
To serve, slice the brisket and put it on a platter with the vegetables. Pour over the sauce and serve with extra sauce on the side.