Whisk 1/2 cup water, the hoisin sauce, honey, ketchup, soy sauce and sambal oelek in a medium bowl. Season the beef all over with the Chinese five-spice powder, salt and pepper. Combine the carrots and turnip in a 6- to 8-quart slow cooker. Add the beef and top with the hoisin mixture; turn to coat. Cover and cook on low until the beef is very tender, 7 1/2 hours.
Remove the vegetables to a bowl using a slotted spoon. Shred the beef with two forks; stir in the sauce to coat. Serve the beef with the vegetables and rice; top with scallions.