Peel the carrots and cut crosswise into 1/2 inch pieces (about 5-6 cups). Peel the potato and cut into small cubes (about 1 1/2 cups).
In a large saucepan over medium-low heat, melt the butter. When hot, add the onion and saute while stirring occasionally, until translucent, 5-6 minutes. Add the garlic and saute while stirring until beginning to change color, 20-30 seconds. Add the carrots, potatoes, and coriander and saute, stirring a couple of times, for about 2-3 minutes. Add the chicken broth, sugar and salt and, over medium heat bring to a simmer, lower the heat, cover partially and continue simmering until vegetables are soft when pierced with the point of a knife, about 20-25 minutes . With a food processor with its metal blade attached, or a blender, process the vegetables in small batches, with their broth, until a smooth puree (soup may be made ahead to this point; cover and refrigerate). Return to the saucepan and add the milk and pepper to taste. Place over medium heat and heat almost to a boil. taste for seasoning and adjust as necessary. If too thick, add more milk. Just before serving, stir in the sherry to your taste.
To serve: ladle into warm soup bowls. Top each bowl with 1-2 spoonfuls sour cream and a generous sprinkling of chopped cilantro.