Make the soup through Step 2 the day before. Reheat, and stir in the lime juice before serving.
Melt butter in a large saucepan over medium-high heat; add onion, and sauté 5 to 7 minutes or until tender. Add garlic; sauté 1 minute. Stir in broth and next 4 ingredients. Bring to a boil; reduce heat to medium-low, and simmer 20 minutes or until potatoes are tender.
Process mixture with a handheld blender until smooth. (If you don't have a handheld blender, cool mixture 10 minutes, and process, in batches, in a regular blender until smooth. Return to saucepan, and proceed with Step )
Cook potato mixture over low heat, stirring occasionally, 5 minutes or until thoroughly heated. Stir in 2 Tbsp. lime juice. Whisk together sour cream and 2 tsp. lime juice. Ladle soup into bowls, and drizzle each serving with sour cream mixture.
I wish I had left out the lime juice. It overpowered the soup. Next time I will either leave out or cut way back.