I like horseradish sauce with my steak, in fact I prefer it to any sort of steak sauce and this is roughly how my sister in law, Chandra, taught me to make it.
Horseradish (not the kind with mayo that's all smooth, this is the REAL thing)
Garlic Salt (or Garlic Powder and Salt)
I started out by putting the horseradish and pepper in the bowl, the smart thing would probably be to start with the sour cream. :-) Especially if your horseradish is pretty potent, that way it's easier to control how intense of a flavor you want.
Sprinkle the pepper and garlic salt
Mix everything together and obviously you'll have to taste test to the level of potency you want and how salty, etc...
The End. (and I basically ate that whole bowl of sauce by myself) :-)