In a medium saucepan over medium heat cook onion, celery, and finely chopped red sweet pepper in butter about 4 minutes or until vegetables are tender, stirring occasionally. Stir in flour, lemongrass or lemon peel, and spices. Add broth and coconut milk all at once. Cook and stir until mixture is slightly thickened and bubbly, about 7 minutes. Cook and stir for 1 minute more. Add peanut butter and soy sauce; stir until well blended and heated through. If desired, top each serving with peanuts, cilantro, and red pepper strips.