Melt butter in a Dutch oven over medium-high heat; add onion, and saute until tender. Stir in chicken broth and next 4 ingredients; bring to a boil. Reduce heat, and simmer 1 hour or until potato is tender. Remove and discard thyme sprigs. Cool slightly.
Process mixture in batches in a blender until smooth, stopping to scrape down sides. Return to Dutch oven, and stir in whipping cream and next 4 ingredients. Cook over low heat, stirring occasionally, until thoroughly heated. Serve with Rum Cream; garnish, if desired.