In a large saucepan heat the vegetable oil over medium heat. Add the mustard seeds. When the seeds begin to pop, add the onions and saute until soft and translucent. Add the carrots, ginger, green chili and curry powder and sauté until the seasonings are fragrant, about 3 minutes.
Add the stock raise the heat to high and bring to a boil. Reduce the heat to medium low and simmer uncovered, until the carrots are tender about 6 minutes.
In a blender, puree the soup until smooth and return to the saucepan. Return the soup to medium heat and reheat gently. Just before serving, stir in the chopped basil and lime juice. Season with salt.
Ladle into individual bowls. Garnish with a drizzle of sour cream and lime zest.