Remove all the skin and bones from the rotisserie chicken and discard. Shred or chop the meat and set aside.
In a large pot or Dutch oven, cook the onion in 2 tblsp of butter until softened. Remove from the pot and set aside.
In the same pot, melt remaining 6 tblsp of butter over moderately low heat and whisk in the flour.
Cook, whisking constantly, for about three minutes.
Stir in the onion.
Gradually add the broth then the half and half, whisking the whole time.
Bring the mixture to a boil, reduce the heat and simmer, stirring occasionally, 5 minutes, or until thickened. (It will be nicely and obviously thick.)
Stir in Tabasco, chili powder, cumin, salt and pepper.
Add beans, chilies, chicken, cheese and milk.
Cook over moderately low heat, stirring occasionally for 20 minutes.
Add sour cream.
May be served immediately but is best served the next day.
Serve with the usual chili garnishes--cilantro, cheese, jalapenos, etc.