Curried Broccoli Soup

In a large saucepan, bring 2 cups of the stock to a boil. Add the onion and simmer over medium heat until the onion is translucent, about 10 minutes. Add the apple and potato and cook until tender, about 20 minutes. Add the broccoli, curry, ginger, and the remaining 3 cups of stock. Simmer until broccoli is tender, about 10 minutes. Purée the soup in batches in a blender. Whisk in the yogurt and adjust seasoning with salt and pepper. To serve, garnish with lemon slices or chopped mint leaves. The soup is good hot or cold.