In a skillet cook onion in 2 tablespoons of butter over moderate heat until softened.
In a 6-8 quart heavy kettle melt remaining 6 tablespoons of butter over moderate low heat and whisk in flour. Cool roux, whisking constantly, 3 minutes. STir in onion and gradually add broth and half and half, whisking constantly. Bring mixture to a boil and simmer, stirring occasionally, 5 minutes, or until thickened. Stir in tabasco sauce, chili powder, cumin, salt, and white pepper. Add beans, chilies, chicken, and <Monteray Jack cheese and cook mixture over moderately low heat, stirring, 20 minutes. Stir in sour cream into chili.
Garnish chili with coriander and serve with salsa.