In A Large Resealable Plastic Bag, Combine The Flour, Salt And Pepper. Add Beef, A Few Pieces At A Time, And Shake To Coat. In A Large Skillet, Brown The Beef In Batches In Oil. Transfer Meat To A Slow Cooker With A Slotted Spoon. In A Bowl, Combine The Soup, Mushrooms, Water, Ketchup And Worcestershire Sauce. Pour Over Meat. Cover And Cook On Low For 8 Hours Or Until Meat Is Tender. Remove Beef With A Slotted Spoon; Keep Warm. Transfer Cooking Liquid To A Saucepan. Combine Cornstarch And Cold Water Until Smooth; Gradually Stir Into Cooking Liquid. Bring To A Boil; Cook And Store For 1-2 Minutes Or Until Thickened. Stir A Small Amount Of Hot Liquid Into Sour Cream. Return All To The Pan; Cook On Low Until Heated Through. Serve Over Meat.