In a 5- to 6-quart slow cooker place baking potatoes and sweet potatoes. Top with beef brisket. In a small bowl combine hoisin sauce, salsa, tapioca, and garlic. Pour sauce mixture over meat; spread evenly.
Cover and cook on low-heat setting for 10 hours or on high-heat setting for 5 to 5-1/2 hours. Remove meat from cooker to a cutting board. Cut across the grain into slices. Serve cooking liquid and potatoes over beef. Makes 8 servings.