Heat the oil over medium high heat. Add the onions, carrots, and garlic, and cook for 5 minutes. Add the sugar, 1/2 teaspoon salt, curry, and cayenne and cook for 1 more minute. Add 5 cups of water, bring to a boil, reduce to a simmer, cover, and cook until carrots are tender, about 5 minutes. Puree soup in batches in a blender or a food processor until smooth. Stir in the lemon juice and viniegar and set aside.
Make the relish:
Combine the cucumber, pistachios, ginger, cilantro, mint, and the remaining salt in a small bowl. Ladle the soup into bowls and top with the relish. Enjoy!