Wrap potatoes in foil; bake at 375° for 1 hour or until tender.
Place rutabaga in a large saucepan; cover with water. Bring to a boil. Cook 5 minutes. Add carrot; cook for 5 minutes. Add parsnip; cook for 5 minutes or until rutabaga is tender. Drain; mash vegetables to desired consistency. Stir in buttermilk and remaining ingredients.
Unwrap potatoes; cut a 1/4-inch slice from the top of each potato; scoop out pulp, leaving a 1/4-inch-thick shell. Add pulp to rutabaga mixture; stir well. Stuff the shells with the potato mixture, and place on a baking sheet. Bake at 375° for 10 minutes or until thoroughly heated.