In a large skillet, heat oil over medium-high heat; saute onion, garlic and ginger until tender, 4-5 minutes. Transfer to a 5-qt. slow cooker., Add all remaining ingredients except cream and green onions. Cook, covered, on low until vegetables are soft, 7-9 hours., Puree soup using an immersion blender, or cool slightly and, in batches, puree in a blender and return to slow cooker. Stir in cream; heat through. If desired, top with green onions.