In a large saucepan, saute onions, celery, carrots and caraway in butter until vegetables are tender. Remove from the heat; stir in flour until well blended. Gradually stir in broth. Return to the heat; bring to a boil. Cook and stir for 2 minutes. Reduce heat; simmer for 10 minutes. Combine sour cream and milk; add about 1 cup broth mixture. Return all to the pan; heat through (do not boil). Season with salt and pepper.