Side

Preheat oven to 400°F.
Fry the chopped onions and thyme sprigs in the olive oil until softened and lightly coloured (about 5 minutes)
Oil a 1.5-litre gratin dish
Layer potatoes in the dish, sprinkling some of the fried onions between layers and finishing with a layer of potatoes
Pour over the vegetable stock and bake for 50–60 minutes until the potatoes are cooked and the top is golden and crisp