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Cover the beans with cold water, add the onion, carrot and herbs. Bring to the boil and cook for 10 minutes, then reduce the heat and simmer for about 30 minutes until tender.
Remove 3-4 tbsp of the nearly cooked beans and set aside for serving. Continue to cook the rest for another 10-20 minutes or so until very soft, topping up with more boiling water if necessary. Drain and discard the vegetables and herbs. Place the beans in a food processor and whizz to a purée until creamy. It may be easier to purée half at a time, ladling in some of the stock to slacken the mixture and get it going. Stop the machine after 2-3 minutes, scrape down with a spatula, correct the seasoning, add a bit more stock and buzz again. The longer you leave it puréeing, the smoother it will become; 4-5 minutes should do it. Return to the same pan. Stir in the stock and seasoning, bring to the boil then simmer for 5 minutes.
Brush the mushrooms to remove any soil then slice down them for the best shape, but not too thinly or they will shrivel when cooking. Heat a non-stick pan until quite hot, add the oil, then the mushrooms, and sauté until they start to colour. Now add the 25g/1oz of butter and continue to sauté until just cooked. Keep the heat in the pan high, as you want to roast rather than boil them. Season and drain.
Rub the soup through a fine sieve with the back of a ladle for a fine velvety texture. Return to the pan and stir in the cream. As the mixture almost comes to the boil, add the truffle oil, if using. (If you add the oil when the mixture is cold, it may split.)
When ready to serve, divide the reserved beans (they don’t need re-heating) and mushrooms between warmed soup cups. Add the knob of butter to the near-boiling soup then froth with a hand blender (do this for 4-5 minutes to get it really light) then slowly pour the soup into the cups from a ladle to three quarters full, wiping the ladle over the edge of the pan so the soup doesn’t drip. Hold back the froth in the pan then scoop that on top of the cups to serve.