Heat a large non-stick pan until hot and add 1 tbsp olive oil. Add the chestnut mushrooms and a pinch of salt, and fry, stirring, until they start to soften and give out liquid. Keep frying until all the moisture has disappeared and the mushrooms have coloured to a dark golden brown. Don’t skimp on this bit as this will ensure you get maximum flavour in the finished sauce. Scoop out the mushrooms then add the onions, garlic, carrots and celery to the same pan with another tbsp of olive oil and stir well. Put on a lid and cook for 10 minutes, stirring now and again, or until the veg has softened. Add the herbs, celery salt, star anise and tomato purée. Drain the porcini mushrooms, keeping the liquid. Chop the porcini well and add to the sauce, along with the strained liquid. Tip the cooked chestnut mushrooms back in.