Heat vegetable oil in a saucepan and fry onion until soft. Stir in chilli powder, ground ginger and garlic cloves. Add vegetable stock, sweet potatoes and button mushrooms and simmer for about 10 minutes until tender. Then add coconut milk and baby spinach and simmer for about 2-3 minutes or until the spinach just starts to wilt. Stir in the juice of a lime or lemon and season. You can sprinkle on some fresh coriander just before serving.