Prepare a medium pot on a low heat and add some oil.
Peel and roughly chop the onions, leek and celery and add to pan. Leave the garlic whole and add to pan. Add the thyme and give it a good stir.
Increase the heat to medium and add some butter.
Peel, wash and roughly cut the potato and add to pan. Leave all veg to cook until softened.
Add cold water to the pan until it barely covers all vegetables. Add bay leaf, vegetable stock cube and bouquet garni. Bring it to the boil and simmer for about 15-20 minutes or until the potato is cooked thoroughly.
Add mushrooms and turn the heat up to bring it back to the boil and simmer for another 5 minutes.
Switch off the hob, remove bay leaf and bouquet garni and blend the soup with a hand mixer. Season with salt and pepper as you go along.