Cut meat into 2 inch strips, salt and pepper, place in a bowl in refrigerator for 2 hrs

Melt half of butter in heavy skillet, cook mushrooms until tender, remove and set aside.

Saute onions until tender.

Melt rest of butter and brown meat on all sides

Blend flour with 1Tbs melted butter in same skillet, gradually add beef boullion stirring until thickened

Add tomato paste, dry mustard and sherry blending after adding each.

Combine sauce, meat, mushrooms and onion

Cook over low heat for 20 min

Blend in sour cream 5min before serving