Fry the onions and thyme sprigs in the oil until softened and lightly coloured (about 5 mins).

2)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.