Fill and boil your kettle. Trim the leeks, rinse out any grit and finely slice them. Trim the celery sticks and finely chop them. Peel and dice the potatoes.

Warm 1 tbsp oil in a large pan on a low heat. Add the veg and the bay leaf. Season with salt and pepper. Pop the lid on the pan. Sweat the veg, keeping the heat low and stirring occasionally, for 15 mins till softened. If the veg stick, turn the heat down more and add a splash of water.

Meanwhile, trim the Brussels sprouts and discard any ragged leaves. Roughly chop them. Tip the vegetable stock powder into a heatproof jug. Add 1 ltr hot water. Stir to dissolve the stock.

Roughly chop the chestnuts. Grate the zest from the lemon. Finely slice the sage leaves.

Add the Brussels sprouts and the veg stock to the softened veg. Leave the lid off the pan (this will ensure the Brussels stay bright green). Turn up the heat. Bring to the boil, then turn the heat down and simmer for 7-8 mins till the Brussels are tender.

Meanwhile, put a frying pan on a medium heat. Add 1 tbsp oil, the chestnuts, lemon zest and sage. Season with salt and pepper. Fry for 4-5 mins, stirring frequently, till the chestnuts have picked up some colour and the pan is aromatic. Take off the heat.

When the veg in the pan are soft, ladle the soup into a blender. Whizz until smooth. Alternatively, use a hand-held blender in the pan. Pour the soup back into the pan. Add the cream and a squeeze of lemon juice. Taste and add more salt, pepper or lemon juice, if you think it needs it. Warm through.

Ladle the soup into warm bowls. Top with the fried chestnuts. Serve straight away.