Heat the oil in a large saucepan over a medium-high heat. Add the onion and cook for 5 mins. Add the mushrooms, increase the heat to high and cook for 10 mins. Add the garlic and cook for 1 min. Remove a large spoonful of the mushrooms and set aside on a plate as garnish.
Pour the stock into the pan and bring to the boil. Remove from the heat and blend until smooth.
Return the soup to the saucepan, add the milk and heat through. Add more water or stock if your soup is very thick. Season to taste and serve in bowls garnished with the reserved mushrooms, a sprinkling of parsley and a drizzle of olive oil.