1. Cut carrots into 1/2-inch-thick slices. Place on a baking sheet with garlic and toss with olive oil and salt and pepper to taste. Roast at 500 degrees 10 minutes.
2. Cut onion into 6 wedges. Add to carrots and garlic and stir to coat with oil. Roast until vegetables are very tender and slightly charred, about 10 minutes.
3. Put carrots, garlic and onion in blender with vegetable broth and puree until smooth, adding more broth if needed, 1 to 2 minutes. Pour into a saucepan and heat over medium heat, 1 to 2 minutes. Stir in yogurt/sour cream, if using, lemon juice.