Place first six ingredients in a 6-qt. stockpot; bring to a boil. Reduce heat; simmer, covered, until vegetables are very tender, 25-30 minutes., In a small skillet, heat butter over medium-high heat; saute onion, garlic, horseradish and ginger until onion is tender. Add to carrot mixture. Puree soup using an immersion blender or cool slightly and puree soup in batches in a blender. Stir in buttermilk; heat through (do not boil). Serve with sour cream and, if desired, dill.