Sweat the onions in a high saucepan with a little oil. Gradually add the pumpkin, potatoes and carrots and sauté briefly. Then season with the spices and deglaze with the vegetable stock.
Let everything simmer on a low flame for about 45 minutes. When everything is done, use the hand mixer to puree into a creamy soup. Serve with a dollop of sour cream, if desired.