Melt the butter gently in a large saucepan. Add the onion, celery, garlic and thyme. Cook for 3 minutes. Sprinkle in the flour, stirring constantly. Keep stirring while adding stock slowly. Increase the heat and add the peanut butter. Reduce the heat, add milk, salt and hot pepper sauce and simmer for 15 minutes. Garnish with cilantro (coriander). Serve hot.