Heat the oven to Gas 6, 200C, fan 180C.
Boil the potatoes for about 5 minutes, or until the slices are soft but not falling apart, drain, cool and set aside
Meanwhile heat the butter and oil in a frying pan and sauté the onion, leek and garlic with a pinch of salt, sugar and pepper, and cook for about 5-10 minutes, until wilted and soft.
In a shallow oven proof dish approx 20cm x 30cm , place half the potatoes in a layer on the bottom of the dish, and season. Add the leek mixture, and spread evenly. Top with the other half of the potato mixture.
Pour over the cream and vegetable stock. Sprinkle over the thyme, some black pepper, and top with the grated cheese. Place in the oven and bake for 30 minutes until golden.
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