Heat the butter in a non-stick frying pan over medium-low heat. Add the leeks and garlic and cook 10 minutes or until softened.
Whisk together thecream cheese and cream until smooth. Whisk in the stock, thyme and seasonings.
Layer half the potato and parsnip into a greased 2½ litre capacity baking dish. Top with leek mixture. Pour over half the cream cheese mixture then top with remaining potato and parsnip. Sprinkle with cheddar cheese and pour over remaining cream cheese mixture.
Bake, covered, at 180°C for 50 minutes, then uncovered for a further 20 minutes, or until vegetables are tender.