Cook the pasta according to the package instructions. Drain, but save about 1/4 cup of the water for the sauce!
Melt the butter in a sauce pan and stir in the flour. Cook until the flour starts to turn slightly brown and smell nutty.
Stir in the heavy cream and sour cream. Using a wire whisk, cook until it is a thick, creamy sauce. Slowly pour in the water from the pasta and continue to stir.
Stir in the cheese and whisk until it is melted.
Add the pasta and half of the walnuts to the pot and toss until the noodles are well coated.
Serve immediately garnished with chives, a little more cheese and the rest of the walnuts.