Pour the stock into a large pan with the cauliflower, potatoes, onion, garlic and curry powder, and bring to the boil. Cover and cook over a low heat until the cauliflower and potatoes are tender, about 20 minutes.
Puree the soup till smooth using an immersion blender or liquidiser. Season to taste with salt and pepper. Serve each bowl with a spoonful of soured cream, chopped coriander and a squeeze of lemon.