1. Boil the vegetables in the water.
2. Strain the stock and add the mushrooms to it.
3. Melt the butter in a pan, add the cornflour and fry briefly.
4. Then add one cup of vegetable stock.
5. Season with salt and pepper.
6. Stir briskly till the stock thickens into a white sauce.
7. Pour the mushroom soup into a large pan and stir in the white sauce.
8. Simmer for 5 minutes. Serve hot, topped with the butter or cream.