Place first 12 ingredients in a 5- or 6-qt. slow cooker; stir to combine. Cook, covered, on low until chicken and vegetables are tender, 6-8 hours., Mix sour cream and flour until smooth; stir into soup. Stir in asparagus. Cook, covered, on high until soup is thickened and asparagus is tender, 20-25 minutes. If desired, serve with green onions.