Preheat the oven to 180ºC / 350ºF / Gas Mark 4.

Take a pie dish or roasting tin roughly 25cm x 20cm and start layering up the sliced vegetables and herbs as follows: one quarter of the potatoes; one quarter of the jerusalem artichoke; one third of the onions, one third of the herbs. Season generously with salt and pepper. Repeat these layers two more times, then finally top with the remainder of the potato and jerusalem artichoke.

Combine the stock and milk in a measuring jug and pour over the vegetables. Finally, cut the butter into small cubes and sprinkle over the top of the gratin.

Bake for 1 hour or until the top is golden and a knife slides through the layers easily.

You can serve this immediately, or it is fine to keep warm for a while, or even chill in the fridge overnight and reheat when ready to serve.