Combine carrots, stock, and salt in soup pot, bring to a boil, reduce heat, and simmer, covered, for 15 minutes. Saute onions, garlic, and cashews in butter until onions are transparent. Puree carrots and stock with sauteed mixture in food processor until smooth. Return to soup pot, whisk in half-and-half and add herbs, stirring until heated through. Serve garnished with dollop of sour cream.