Celeriac, Walnut and Devon Blue Gratin

Fill and boil your kettle, to make up the stock, and pre-heat your oven to 190°C (Gas Mark 5). Cut the peeled celeriac into quarters, then slice these as thinly as you can (a mandolin or the slicing blade of a food processor is easiest). Put the slices into an oven dish and drizzle over the oil, then add a good pinch of sea salt and a few grinds of the pepper mill and toss everything together. Pour the stock into a mixing bowl and add the lentils, then stir in the cream. Pour this mixture over the celeriac, then put the dish into the hot oven for 40 minutes. Next, remove the dish from the oven and scatter the walnuts and the crumbled cheese across the top, then put back into the oven for another 15 minutes, or until crisp and golden. Serve hot, with a few salad leaves.