Melt butter in a large saucepan. Add the shallots, onion, and celery; cook over low heat about 6 minutes (keep stirring).
Add vegetable (or chicken) stock, nutmeg, and sweet potatoes; bring mixture to a boil, lower the heat, and simmer for about 30 minutes. When the potatoes seem tender, put the mixture into a food processor or blender and mix until smooth.
Put mixture into a saucepan and add the half and half and sherry; stir to mix and smooth out. Season with pepper and continue to heat but don't bring to a full boil.
Place pecans in a small dish and have each diner sprinkle the chopped walnuts in the filled soup bowl.