Heat oil in large saucepan over medium heat. Add onion and garlic; saute until onion is transparent.
Add chicken, tomatoes, peanut butter, salt and red pepper flakes. Heat, stirring occasionally, until peanut butter is melted. Blend in buttermilk and cook until heated through. Stir in cilantro or parsley.
To serve, ladle soup in bowls and garnish with cilantro and chopped peanuts.