In a large saucepan, heat the olive oil over medium heat. Add the mustard seed. When the seeds just start to pop, after about 1 minute, add the onion and saute until soft and translucent, about 4 minutes. Add the carrots, ginger, jalapeno and curry powder and saute until the seasonings are fragrant, about 3 minutes.
Add 3 cups of 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 or food processor, puree the soup in batches until smooth and return to the saucepan. Stir in the remaining 2 cups stock. Return the soup to medium heat and reheat gently. Just before serving, stir in the chopped cilantro and lime juice. Season with the salt, if desired.
Ladle into warmed individual bowls. Garnish with a drizzle of yogurt, a sprinkle of lime zest and cilantro leaves.