TIP: to make slicing easier, place beef in freezer for one hour. If already frozen, slice it before it is completely thawed.
Slice beef into very thin strips.
In a medium skillet, over medium-high, heat 1 TBSP butter and 1 tsp. of the oil. Add beef in small batches and cook until browned, stirring often. Remove cooked batches of beef with a slotted spoon, keeping oils in the pan. Set beef aside and cover to keep warm.
Reduce heat to medium. Add remaining butter and oil. Add mushrooms and onion, and cook until tender, about 5 - 7 minutes.
Add soup, water and dijon mustard. Heat to a slight simmer. Return beef to pan, juices and all, and heat through; let simmer while preparing rice or egg noodles.
Serve over rice or noodles, with a side of broccoli or asparagus!