Trim fat from meat.
Reserve fat.
Cut meat diagonally across grain in strips, 1/4 inch wide. (Strips should be very thin.)
Heat fat in skillet or chafing dish.
When you have about 3 T. melted fat, remove trimmings then brown meat quickly.
Add water and mushrooms (including liquid).
Stir in soup mix.
Heat just to boiling.
Blend sour cream and flour, and add to the rest.
Cook and stir till mixture thickens - sauce will be thin.
(If using chafing dish, keep warm over hot water.)
Serve over hot noodles or rice.

Note: If fat coating is thin, add butter or margarine to make 3 T.