Sweat the onion slices, vegetables and potatoes in the oil. Add a pinch of sugar. Deglaze with the wine and simmer a little. Top up with vegetable stock and milk, bring to the boil and simmer for about 20 minutes over low heat. Puree the soup and strain through a sieve if necessary. Bring to the boil again and stir in the whipped cream. To taste.
If you like, you can garnish the soup with fried onion slices.