1. Preheat oven to 400°F (200°C).
2. On a large roasting pan, combine carrots, mushrooms, onion, and whole garlic cloves. Drizzle with oil and toss to coat.
3. Roast vegetables for 25-30 minutes, stirring halfway through, until carrots are tender and mushrooms are golden brown.
4. Remove from oven. Squeeze roasted garlic from skins and discard skins.
5. Transfer roasted vegetables to a large soup pot. Add 4 cups of broth and sage.
6. Using an immersion blender, blend until smooth. Alternatively, carefully transfer to a blender in batches.
7. Add additional broth if needed to reach desired consistency.
8. Stir in sour cream and heat through.
9. Season with salt and pepper to taste. Serve hot.