Heat the oil in a frying pan. Sauté the leek for approx. 5 mins., then remove from the pan. Fry the sausage meat in batches for approx. 3 mins. per batch. Return the leek and sausage to the pan, pour in the wine, season. Transfer the sausage mixture to the prepared tin.
Arrange the potato slices in rows on the sausage mixture. Combine the milk, cream and garlic, season. Pour the sauce over the potatoes and top with cheese and thyme.
Approx. 45 mins. in the centre of an oven preheated to 200°C.