In a large saucepan, heat oil over medium heat. Add onions and garlic; cook, stirring occasionally, until softened about 7 minutes.
Add carrots cumin, pepper flakes, and salt. Stir to coat. Cook, stirring occasionally, for 10 minutes longer.
Add broth and water; bring to a simmer until carrots are tender, 10-12 minutes. Transfer soup, in batches, to a blender and puree until smooth. Return to the pan and bring to a simmer to heat through. Adjust seasonings and serve hot with a teaspoon of sour cream per serving if desired.