Heat the oil in a large pan. Tip in the onions and fry for 10 mins until golden. Meanwhile, pour 1.2 litres boiling water over the dried mushrooms and stir in the bouillon.
Add the fresh mushrooms and garlic to the pan with the potatoes, thyme and carrots, and continue to fry until the mushrooms soften and start to brown.
Pour in the dried mushrooms and stock, cover the pan and leave to simmer for 20 mins. Stir in the parsley and plenty of pepper. Ladle into bowls and serve each portion topped with 2 tbsp yogurt and a quarter of the walnuts. The rest can be chilled and reheated the next day.