Peel and finely dice the onion. Either pre-cook the chestnuts or use ready-to-eat ones from the vacuum bag. Melt the butter in a soup pot. Chop chestnuts a bit and add to butter with diced onion and sauté a bit.
When the onion pieces have browned a bit, pour in soup, add salt and pepper. Simmer the soup for about 15 minutes.
Meanwhile, whip a third com whipped cream, add the rest to the soup. Puree the soup and serve each with a dollop of whipped cream and some parsley.