1 tbs powdered mustard
1 ½ tsp sugar
4 tbs vegetable oil
2 medium onions, thinly sliced
1 lb mushrooms, thinly sliced
2 lb beef tenderloin (or other lean, tender cut)
2 tsp salt
1 tsp freshly ground black pepper
2 cups sour cream
In a small bowl, combine mustard, sugar, a pinch of the salt and 1 tbs water to form a thick paste. Set aside to allow flavours to develop while preparing the remaining ingredients.
Heat half of the oil in a heavy skillet over high heat until hot but not smoking. Add onions and mushrooms, cover pan and reduce heat to low. Cook 20-30 minutes, stirring occasionally, until vegetables are soft. Drain off liquid and return to skillet, covering to keep warm.
Cut beef into ¼-inch strips. Heat 2 tbs oil in a heavy skillet over high heat until hot but not smoking. Drop in half of the meat, tossing constantly for 2 minutes, until lightly browned. Using tongs, transfer meat to skillet with vegetables and repeat searing with remaining beef, transferring to vegetables when done.
To the combined vegetables and beef, add salt, pepper and mustard paste. Add sour cream, a few tbs at a time. Simmer over low heat for 2-3 minutes. Serve over warm egg noodles.