Peel potatoes and parsnips; cut into 1/2-inch chunks. Cook in a Dutch oven or large saucepan in boiling salted water until tender, about 25 to 30 minutes. Drain.
Meanwhile, preheat oven to 350 degrees F. Cut about 1/2 inch off tip of garlic head, and discard. Place garlic in a custard cup. Drizzle with olive oil. Cover with foil and bake in the preheated oven for 20 minutes or until tender when pierced with the tip of a sharp knife. Let cool. Squeeze garlic pulp out of peels. Mash with a fork, and set aside.
Press the potatoes and parsnips through a potato ricer or food mill (or mash with a potato masher), and return them to the Dutch oven.
Heat milk and butter in a small saucepan just until boiling. Stir into the potato mixture. Stir in mashed garlic, the 3/4 teaspoon salt, and the 1/8 teaspoon pepper. Heat through. Season to taste with additional salt and pepper. Serve immediately. Makes 14 servings.