Melt butter in a large saucepan over medium heat; add onion and celery, and sauté 5 minutes. Add garlic; sauté 2 minutes. Stir in flour, curry powder, and red pepper until smooth; cook, stirring constantly, 1 minute. Add broth, and bring to a boil; reduce heat to low, and simmer 20 minutes.
Stir in peanut butter and half-and-half; cook, stirring constantly, 3 to 4 minutes or until thoroughly heated. Cool slightly.
Process mixture, in batches, in a food processor or blender until smooth. Garnish, if desired. Serve immediately.