1. Melt the butter in a 6-quart (or larger) soup pot over high heat. Add the carrots, onions, ginger, thyme bundle, salt, and white pepper, and cook for 2 minutes. Then add the water, cover the pot, and bring to a boil. Remove the cover, reduce the heat to medium-low, and simmer for 15 minutes.
2. Remove the pot from the heat, and remove the thyme bundle. Blend the soup until it is completely smooth, using an immersion blender or in three batches in a blender (see Note).
3. Transfer the pureed soup to a 4-quart pot or other serving dish. Stir to combine, and adjust the seasoning to taste. Serve hot, garnished with a dollop of sour cream if desired.
Note: Please use caution when blending hot liquids; blend only small amounts at a time, with the blender tightly covered and a kitchen towel held over the top.
Yield: 2 1⁄2 quarts, about 6 servings