1. Lightly grease an ovenproof dish (8 cup capacity).
2. Heat oil in a large frying pan over medium heat. Add bacon, leek and garlic. Cook, stirring occasionally, for about 10 minutes, or until leek is soft.
3. Place undiluted soup and cream in a large jug. Whisk until smooth.
4. Arrange a third of the potato slices over base of the prepared dish. Top with a third of the bacon mixture and a third of the soup mixture. Sprinkle with 1⁄3 cup of the cheese. Season with salt and pepper. Repeat layering with potato slices, bacon mixture, soup mixture, cheese and salt and pepper. Cover tightly with foil and place dish on an oven tray.
5. Cook in a moderate oven (180C) for about 1 hour, or until potatoes are tender. Remove foil. Sprinkle with extra cheese. Turn oven up to 200C. Cook for 15 minutes, or until cheese is golden. Remove.
6. Stand gratin for 5 minutes before serving.

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