Heat the oven to 200C. Grind the cumin and coriander seeds, chilli flakes and a pinch of salt in a pestle and mortar. Put parsnips in a roasting tray, drizzle with olive oil and scatter over the spice mixture. Toss the parsnips well to coat them. Roast in the oven for about half an hour or until soft and golden.
Meanwhile, fry the onion and celery in a pan until soft and slightly coloured. Add the potato and lentils and mix through. Add the stock and the roasted parsnips. Simmer until lentils are cooked, about 20 mins. Use a stick blender to whizz until smooth.