Trim all excess fat from roast; brown and drain if using chuck or another highly marbled cut.
Combine 1/4 cup flour, the salt and pepper. Coat meat with the flour mixture. Place all vegetables except mushrooms in Crock-Pot and top with roast (cut roast in half, if necessary, to fit easily). Spread mushrooms evenly over top of roast. Cover and cook on Low for 10 to 12 hours. If desired, turn to High during last hour to soften vegetables and make a gravy.
To thicken gravy, make a smooth paste of the 3 tablespoons flour and the water and stir into crock pot. Season to taste before serving.