1. In a large (5-quart) saucepan, heat oil over medium; add onions and cook, stirring occasionally, until translucent, about 5 minutes. Add carrots, sweet potatoes, ginger, broth, and 4 cups water. Season with salt and pepper.

2. Bring to a boil, reduce to a simmer, and cook until vegetables are tender, about 20 minutes. Working in batches, puree mixture in a blender until smooth; add sugar and lemon juice to taste. Season with salt and pepper, if necessary. (If freezing, proceed to step 3.) Return soup to pan; heat over medium, stirring in cream, if desired.

3. To freeze: Cool to room temperature. Divide among airtight containers (leaving 1 inch space at top), and freeze up to 3 months. To heat, place container upside down under hot tap water to release block of soup; place in a medium saucepan. Cover, and cook over medium-low, adding water to thin, if necessary. Stir in cream, if desired.