Slice the onions. In a large saucepan, cook onions in butter over medium heat for 5 minutes. Add the butternut (cut un cubes), the garlic, the chestnut (keep a little bit for decoration) and the stock (don't put too much, it should barely cover the veggies).
Cover and simmer for about 20 minutes. Check that your butternut is tender, otherwise cook a few more minutes. Blend everything.
Heat a pan and melt a spoon of butter. Roast the remaining chestnut in the butter for a few minutes.
Serve the soup with the roasted chestnut on top ot if.