Place first 3 ingredients in a Dutch oven. Cover with water to 2 inches above vegetables, and bring to a boil. Reduce heat, and simmer for 25 minutes or until rutabaga is very tender; drain. Return pan to heat. Add milk and butter to pan; cook for 1 minute or until thoroughly heated. Add rutabaga mixture to pan; mash with a potato masher to desired consistency. Stir in salt, mustard, and pepper.