1. Melt butter in a Dutch oven over medium heat; add onion and celery, and sauté 6 minutes or until tender. Add garlic, and sauté 1 minute.
2. Add broth, potatoes, and sage. Bring to a boil; cover, reduce heat to low, and simmer 20 minutes or until potatoes are very tender. Remove from heat. Uncover and let cool 10 minutes.
3. Process potato mixture, in batches, in a food processor or blender until smooth. Return mixture to Dutch oven; whisk in peanut butter and next 3 ingredients.
4. Cook over medium-low heat, stirring often, 3 to 4 minutes or until thoroughly heated. Garnish, if desired.