In a Dutch oven combine turnips, carrots, parsnips and onion. Add enough water to cover. Bring to boiling; reduce heat. Cover and simmer for 25 to 30 minutes or until vegetables are tender. Drain well. Return vegetables to the hot pan. Mash vegetables with a potato masher while adding the butter, milk, salt and pepper. Vegetables should still have plenty of texture and not be completely smooth. Divide among four serving plates.