1. In a skillet, heat oil over medium heat. Add onion and sauté until softened, about 6 minutes. Add curry paste and sauté for 1 minute.

2. In a food processor or blender, pureé onion mixture, mangos, brown sugar, ginger and salt until smooth. Add coconut milk, water, yogurt and lime juice; process to combine. Transfer to a bowl, cover and refrigerate until cold, about 3 hours, or overnight. Taste and adjust seasoning with salt, if necessary.

3. Prepare the relish: In a small bowl, combine mango, brown sugar and basil. Let stand for 15 minutes.

4. Ladle soup into chilled bowls and top each with a dollop of mango relish.