Melt butter in a skillet over medium high heat.
Add onions and saute until glassy then add mushrooms and saute for 5 more minutes.
Add sour cream and cook for 5 to 10 minutes (depends on how thick you like the sauce)
Add garlic and cook for 1 more minute - optional.
Add dill or parsley, stir well and cook for 1 more minute - optional.
Sprinkle hot chili pepper flakes when serving - optional.
Serve as sauce on steaks, a side dish or a main dish with salad.