Peel the potatoes and cut them into little cubes.
Put olive oil in to a non-stick pan and fry the onion, being careful not to burn it. After two or three minutes add the tomato paste and the mushrooms and let them cook for another two minutes.
Add hot stock and the potatoes and boil for 20 minutes.
Reduce the heat and add the fresh cream, letting the soup cook for another two minutes.
Add some fresh grated pepper, if wished, and serve with croutons seasoned with salt and olive oil.