In a large skillet, heat oil over medium heat. Add onion; saute until tender, 4-5 minutes. Add garlic; cook 1 minute longer. Transfer to a 6-qt. slow cooker. Add next 5 ingredients and 5 cups broth to slow cooker. Cook, covered, on low until vegetables are soft, 6-8 hours., Puree soup using an immersion blender. Or cool slightly and puree soup in batches in a blender; return to slow cooker. Stir in additional broth to reach desired consistency; heat through. If desired, top servings with sour cream.