Mixed mushroom soup

This in one of my long-term favourites, hearty, yes, but earthy, satisfying and bursting with flavour!

2 tablespoons extra virgin olive oil
1 onion, finely chopped
3 sticks celery, finely chopped
1 garlic clove, crushed
500 g mixed mushrooms, chopped roughly (I prefer a majority of Swiss Brown, with a handful of button mushrooms plus some exotics such as shitake etc.)
1 large potato
1 litre vegetable stock
2 tablespoons freshly chopped parsley or coriander
Salt and freshly ground pepper
Thick plain yoghurt to serve (optional)

Method

1. Heat olive oil in large saucepan, add the onion, celery and garlic and
   fry for 3–5 minutes until softened.
2. Stir in the mushrooms and continue to cook for another 5 minutes. Add the potatoes and the stock and bring to the boil. Reduce heat and leave to simmer gently for 30 minutes.
3. After slightly cooling the soup, blend until smooth, but leaving some chunky portions of mushrooms to add to the blended mix prior to serving.
4. Sprinkle with parsley or coriander and serve with a dollop of horseradish cream or thick yoghurt.