1 - Heat oven to 200C/fan 180C/gas 6.

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

3 - Spread a layer of potatoes over the base of a 1.

4 - 5-litre oiled gratin dish.

5 - Sprinkle over a few onions (see picture, above) and continue layering, finishing with a layer of potatoes.