Melt the butter in a large pot over medium-low heat. Add in onion, garlic and mushrooms, and stir-fry, stirring occasionally, until softened, about 5 minutes.
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 or use a stick blender to blend until smooth and creamy.
Return the blended mushroom mixture into the pot, pour in double cream and season to taste with salt and black pepper. Continue to cook and reduce for another 5 minutes, or until you reach the desired thickness.
Pour in the reserved cooked mushrooms, sprinkle with fresh parsley and season with freshly cracked black pepper.