In a large saucepan heat the oil over a medium heat. Add the onion, carrot, celery, chorizo and a pinch of salt and fry for about 20 minutes, stirring occasionally, until everything caramelises and turns quite brown. This gives the soup a wonderfully rich colour and taste. Now add the garlic, cumin, thyme and chilli and cook for 1 more minute, followed by the tomato and, after about 2 minutes, the chestnuts. Give everything a good stir, then add the saffron-infused liquid, and the water, and simmer for about 10 minutes. Remove from the heat and mash by hand (with a potato masher) until almost smooth but still with a little bit of texture. Season with salt and pepper.