In a 6-qt. stockpot, heat butter and oil over medium heat. Stir in vegetables and seasonings; cook until vegetables are tender, 8-10 minutes, stirring occasionally., Stir in rice and broth; bring to a boil. Reduce heat; simmer, covered, until rice is tender, 40-45 minutes, stirring occasionally., Puree soup using an immersion blender, or cool slightly and puree soup in batches in a blender. Return to pot and heat through. If desired, serve with sour cream and croutons.