Preheat oven to 220°C or 200°C fan forced. Scatter cauliflower over base of a greased roasting pan and mushrooms into another pan. Drizzle 1½ tablespoons of oil over each pan and then sprinkle each pan with 1½ teaspoons curry powder, season and turn to coat. Place cauliflower on top shelf and mushrooms underneath in oven, roast both for 20-25 minutes until tender.
Heat remaining oil in a large saucepan over medium heat. Add leek. Cook, stirring occasionally for 5 minutes or until soft. Pour over stock and bring to the boil. Add roasted cauliflower and mushrooms and return to the boil.
Remove from the heat. Blend or process soup, in batches, to desired consistency. Stir in parsley, taste and adjust seasoning. Ladle into warm bowls. Serve with cheese toasts if desired.