Melt butter and oil in a pan on a low heat. Add onion until soft but not brown. Appox 10 mins.
Increase heat to high and add the mushrooms. Cook for about 3 minutes, stirring occasionally, until softened.
Then add flour and cook through for 5 minutes stirring constantly to prevent browning. Slowly add the stock stirring constantly to prevent lumps. Add the thyme and simmer for 10 minutes. Add salt and pepper to taste and then blend using a hand blender until you have a smooth consistency.
Reheat the blended soup in the pot if needed, then serve with a drizzle of cream and fresh parsley.