Peel potatoes and parsnips; cut into 1/2-inch chunks. In a covered Dutch oven or large saucepan, cook potato and parsnip for 25 minutes in enough boiling salted water to cover; drain.
Meanwhile, preheat oven to 350 degrees F. Peel away the dry outer layers of skin from garlic head, leaving skins and cloves intact. Cut about 1/2 inch off the pointed top portion of the garlic head and discard. Place garlic, cut side up, in a custard cup. Drizzle with olive oil. Cover with foil and bake about 20 minutes or until garlic is tender when pierced with the tip of a sharp knife. Cool. Press garlic pulp from each clove with your fingers and place in custard cup; mash with a fork. Set aside.
Mash potato and parsnip with a potato masher or ricer or beat with an electric mixer on low speed. In a small saucepan, combine milk and butter. Bring just to boiling. Stir milk mixture into potato mixture. Stir in mashed garlic pulp, salt, and pepper. Heat through. Makes 14 servings.