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Heat oven to 200C/fan 180C/gas 6. Fry the onions and thyme sprigs in the oil until softened and lightly coloured (about 5 mins).
Spread a layer of potatoes over the base of a 1.5-litre oiled gratin dish. Sprinkle over a few onions (see picture, above) and continue layering, finishing with a layer of potatoes. Pour over the stock and bake for 50-60 mins until the potatoes are cooked and the top is golden and crisp.