Peel, quarter and thinly slice the sweet potatoes. Chop onion finely. Heat butter in a pot. Sauté sweet potatoes and onion. Season with salt and 1 pinch of sugar. Deglaze with white wine.
Grate the orange peel and add to the sweet potatoes with vegetable stock. Put the lid on the pot and let everything boil for 12-15 minutes.
Peel the orange so that the white skin is completely removed. Cut out the fillets with a knife between the parting skins and collect the juice.
Cut the orange fillets into small pieces, add to the soup with the cream and cook for another 3 minutes. Puree the soup finely with a hand blender. Season to taste with salt and pepper. Serve immediately.