Cook onion, celery, carrot and red pepper in butter until softened, about 5 minutes. Add the lemon grass, red chili, stir and cook for 1 minute. Stir in the flour, and let cook for 1 minute, stirring all the time.
Add chicken broth, coconut milk and soy sauce, and stir to combine well. Add the peanut butter. Keep stirring until well combined (the peanut butter will go kind of stringy, but it will melt in after a bit). Heat until hot and bubbly.
Serve with chopped fresh roasted peanuts and green onions as a garnish.