Sauté onion in 2 tablespoon of butter until softened.
In a large pot, melt remaining 6 tablespoon of butter over moderately low heat and whisk in flour. Cook the roux, whisking constantly, for three minutes.
Stir in the onion; gradually add the broth, milk and whipping cream, whisking the whole time. Bring mixture to a boil; simmer, stirring occasionally until thickened.
Stir in Tabasco, chili powder, cumin, salt and pepper. Add beans, green chilies, chicken and cheese; cook over moderately low heat, stirring occasionally for 20 minutes.
Add sour cream immediately before serving. Optional: can be served with cilantro, cheese, jalapenos, tomatoes, etc. as garnishes.