Preheat oven to 425°. Cut off pointed end of garlic bulb; place bulb on a piece of aluminum foil. Fold to seal. Bake 30 minutes; cool 10 minutes.
Meanwhile, bring rutabaga, celery root, and salted water to cover to a boil in a Dutch oven, and boil 15 minutes or until tender. Drain. Bring potatoes and salted water to cover to a boil in a 4-qt. saucepan, and boil 15 minutes or until tender. Drain. Combine rutabaga mixture and potatoes in Dutch oven.
Cook milk in a small saucepan over low heat 3 to 5 minutes or until thoroughly heated.
Add butter, next 2 ingredients, and hot milk to rutabaga mixture. Squeeze pulp from 2 roasted garlic cloves into mixture; reserve remaining garlic for another use. Mash vegetable mixture with a potato masher until light and fluffy. (Use a food processor for a silkier texture.) Add chives, if desired. Serve immediately.