1. 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.
2. 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 3.)
3. 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.