In a large pot, met the butter over medium-low heat. Add in onion, garlic and mushrooms, and stir-fry, stirring occasionally, until soften, about 5 min.Sprinkle the flour into the pot, and stir to heavenly coat the mushrooms. Season to taste.Pour in the stock and bring to a boil, then reduce the heat and simmer until slightly reduced, about 5 min.Reserve ¼ cup of mushrooms and set aside. Transfer the remaining mixture into a food processor and pulse until creamy and smooth.Return the mixture into the pot, pour in double cream and season to taste. Continue to cook and reduce for another 5 min, or until desired thickness is reached.Pour in the reserved cooked mushrooms, sprinkle with fresh parsley and season with freshly cracked black pepper.Store in an airtight container for up to 2 days.