Rub inner bowl of slow cooker with half the butter. Add milk, sugar and salt; stir until combined. Stir in rice and dot with remaining butter.
Cover slow cooker with lid and cook on HIGH for 2 1/2 to 3 hours or until rice is very tender and mixture is thick and creamy.
Stir in orange zest and vanilla. Serve warm, or cool in refrigerator until well chilled and serve cold.
Sprinkle with nutmeg before serving.
Stir in 1/2 cup raisins in the last 30 minutes of cooking if desired.
Alternatively, cook pudding on LOW in slow cooker for 5 to 6 hours.