Peel the sweet potatoes and carrots and cut into small cubes. Cut the leek into rings. Clean and halve the mushrooms.
Heat the oil in a saucepan, add the carrots, potatoes and leeks and cook for 3 minutes. Add the mushrooms and cook for another 2 minutes. Pour 450 ml of vegetable stock. Bring everything to a boil while stirring. Turn down the gear and cook over low to medium heat for about 10 minutes, stirring occasionally.