I deflaze the pan I seared the steak in with about a cup of beef stock, then add a spoonfun of Dijon mustard and bring it to a boil. I wait to add a quarter cup of sour cream until things have reduced by about half then stir like crazy to incorporate it into the sauce.

I also let the pasta finish cooking in the sauce, which allows it to soak in the flavors more than if it was just creamed at the last second (dried egg pappardelle is a sauce sponge). That final addition of gelatinized starch from the boiling process is what keeps the sauce stuck to the noodles and not the plate.