1/4 lb. (1 stick) sweet butter
1 large yellow onion, thinly sliced
2-lbs. filet mignon
3/4 cup flour
1 lb. fresh mushrooms (sliced lengthwise)
2 envelopes George WashingtonRich Brown Seasoning Broth
1 tsp. salt, 1 tsp. Russian- style mustard
1 cup heavy cream
1 Tbs. sour cream.
Melt butter in large skillet and saute onion until soft but not brown. Remove to oven-proof baking dish. Saute mushrooms briefly in same skillet and remove them to dish with onions.
Slice beef across the grain into 1/4 inch rounds. Lay each round on a board and slice with the grain into 1/4-inch wide strips. Transfer meat to a bowl, add flour and mix well. Remove meat to skillet in which the onions and mushrooms were sauteed, and, on medium heat, cook, stirring, until meat is gray. Add mustard, seasoning broth, 1 cup water, salt and heavy cream.
Mix with onions and mushrooms in baking dish and bake in oven for 20 minutes at 400-degrees. Just before serving, stir in sour cream, and serve from baking dish.