5 cups (or more) low-salt chicken broth or vegetable broth
2 teaspoons fresh lime juice
Plain yogurt (for garnish)
Grind coriander and mustard seeds in spice mill to fine powder. Heat oil in heavy large pot over medium-high heat. Add ground seeds and curry powder; stir 1 minute. Add ginger; stir 1 minute. Add next 3 ingredients. Sprinkle with salt and pepper; sauté until onions begin to soften, about 3 minutes. Add 5 cups broth; bring to boil. Reduce heat to medium-low; simmer uncovered until carrots are tender, about 30 minutes. Cool slightly. Working in batches, puree in blender until smooth. Return soup to pot. Add more broth by 1/4 cupfuls if too thick. Stir in lime juice; season with salt and pepper. DO AHEAD: Can be made 1 day ahead. Cool slightly. Chill uncovered until cold, then cover and keep chilled. Rewarm before serving.
Ladle soup into bowls. Garnish with yogurt and serve.