Indian Spiced Carrot Soup

2 teaspoons coriander seeds (or ground coriander)
1 teaspoon mustard seeds (or ground mustard)
1 teaspoon curry powder
2 tablespoons fresh ginger, minced
2 cups chopped onion
1.5 pounds carrots, peeled and chopped
grated peel of 1-2 limes
5 cups (or more) low-salt broth
juice of 1-2 limes
plain yogurt (for garnish)

If using whole spices, grind to a fine powder.

Heat a small amount of oil in a heavy pot over medium heat. Add spices and sauté for 1 minute. Add ginger, cook for 1 minute more. Add onion, carrot, and lime zest. Sprinkle with salt and pepper and sauté until onion begins to soften, about 3 minutes. Add broth, bring to a boil. Reduce heat to medium-low and simmer uncovered until the carrots are quite soft, about 30 mintues. Puree until smooth. Add lime juice to taste and season with additional salt and pepper.