To make the soup, begin by cutting the onion, and slowly cook it in a copper pot with some olive oil. Add the crushed garlic cloves and cook for 2 minutes. Add 1/2 of the chicken broth and cook onions until they are transparent. Add the remaining broth and the steamed, peeled, and quartered potatoes, and let simmer over low heat for 10 minutes.
Boil the asparagus in salted water for 2 minutes. Add the cooked asparagus to the potato and onion mixture. Blend mixture in a food processor until smooth, strain through a chinois or cheesecloth, set aside and keep warm.
To prepare the foam, steam the leek and set aside. Saute the shallots and the onion with 1 tablespoons olive oil in a saucepan until translucent. Add the heavy cream and broth, and then add the leek and the asparagus. When everything is cooked sufficiently, approximately 10 minutes, put everything into the food processor. Blend until all of the ingredients are well pureed. Pass through the chinois several times until you have a perfectly creamy sauce. Let the mixture cool, and place into a siphon with a double charge of gas. This will help you create the foam.
To serve, pour the warm velvet potato soup into large cappuccino cups. Use the siphon to create the foam top, like a real cappuccino. Finish with a drizzle of aged balsamic vinegar and extra virgin olive oil.
Recipe courtesy of Massimo Bottura, Osteria Francescana, Modena MO, Italy