Preheat oven 200°C fan forced. Combine mince, onion, breadcrumbs, half the parsley and egg in a large bowl. Season with salt and pepper. Mix well. Roll tablespoons of mixture into balls. Place in a single layer on a lightly greased baking tray. Spray with oil and bake 10 minutes until light golden and cooked through.
Melt the butter in a large saucepan over medium-high heat until butter begins to foam. Add leeks and garlic and cook, stirring often, for 3–4 minutes or until soft.
Add the mushrooms. Cover and cook, for 10 minutes or until mushrooms are tender. Add flour and cook, stirring constantly, for 2 minutes. Gradually add stock, stirring constantly to prevent lumps. Bring to the boil, reduce heat to medium-low and simmer, uncovered, stirring occasionally for 10 minutes.
Remove the soup from the heat and blend or process to desired consistency. Return to the saucepan, stir in cream if using. Add the meatballs and stir over medium-low heat, without boiling, until hot. Season with salt and pepper, sprinkle with remaining parsley. Serve with toasted bread.