Chop the onion and fry it in oil, add the flour and sauté it lightly. Mix 8 1/3 cups (2 liters) of water with the stock cubes. Wash the mushrooms well, chop them very finely or best just blend them.
Grate the carrots and potatoes coarsely and add them to the soup along with the mushrooms. Flavor with spices. The soup should boil for 40 minutes, then remove it from the stove, add the cream, bring it to a boil again and remove it from the heat.