1.Stir Together Water, Onion Soup Mix, And Au Jus Mix In A Slow Cooker. Add Beef, Cover, And Cook On High For 5 Hours, Or Until Tender. When The Beef Is Tender, Shred The Meat With Two Forks, And Continue Cooking On Low 1 To 2 Hours. 2.To Serve, Open The Hoagie Buns, And Spoon Meat Mixture Into The Center. Sprinkle With Mozzarella Cheese And Serve.