5 T butter, divided
2 T tomato paste
Melt 2 T butter in a heavy skillet, and saute mushrooms until tender. Remove and set aside.
Melt more butter, if needed, and saute onion in same pan until caramelized. Remove and add to mushrooms.
Add at least another T butter to skillet, then brown meat on both sides in small batches.
(Add butter when needed.) Remove beef and set aside with mushrooms and onion.
Add 2 T flour to skillet and mix with meat drippings and left-over butter. Brown it well, mixing constantly. (If you prefer to use arrowroot powder, add 2T arrowroot mixed with 2T water after simmer.)
Slowly add 2 c beef stock to form a smooth gravy.
Add tomato paste and mustard, blending well.
Next, add meat, mushrooms and onion and let simmer on low heat, covered, for about 20 minutes. (Add 2T arrowroot + 2T water now, if desired)
Five minutes before serving, add the sour cream and blend thoroughly. Season with sea salt and black pepper, if desired.
Serve over long grain rice or mashed potatoes!