Melt the butter in a large saucepan over medium-high heat until butter begins to foam. Add the leeks and garlic. Cook, stirring often, for 3–4 minutes or until leek is soft.
Add the mushrooms. Cover and cook, stirring occasionally, for 10 minutes or until mushrooms are tender. Sprinkle the flour over the mushrooms and cook, stirring constantly, for 2 minutes.
Remove the pan from the heat. Gradually add the stock, stirring constantly. Return saucepan to medium-high heat. Cover and bring to the boil, stirring occasionally. Reduce heat to medium-low. Simmer, uncovered, for 10 minutes or until thickened slightly.
Remove from the heat. Blend or process soup, in batches, to your desired consistency. Stir in the cream and parsley. Return to medium-low heat. Cook, stirring, without boiling, until hot. Season with salt and pepper. Serve with crusty bread.