In a 3-1/2- or 4-quart slow cooker, combine onion, carrots, and turnips. Top with beef. In a medium bowl, whisk together wine, tomato paste, espresso coffee powder, brown sugar, thyme, salt, and pepper. Pour over all in cooker.
Cover and cook on low-heat setting for 8 to 9 hours or on high-heat setting for 4 to 4-1/2 hours. Serve beef mixture over mashed sweet potatoes. If desired, season with additional salt and coarsely ground black pepper. Makes 6 servings.