Heat oil in a large pot over medium-high heat; sauté onions until soft. Add garlic, ginger and curry powder and cook, stirring for 1 minute. Add potatoes and squash and cook, stirring for 10 minutes.
Add broth and iced tea, stirring to loosen any browned bits from the bottom of the pot. Bring to a boil; reduce heat, cover and simmer until vegetables are soft, about 15-20 minutes.
In small batches, carefully purée mixture in a blender until smooth. Season with salt and pepper to taste. Garnish with sour cream. Serves 6.