Slice beef against the grain on a diagonal bias into bite-size strips. In a bowl, season meat with fresh ground pepper and porcini salt. Toss with enough flour to coat meat on all sides.
In a large skillet (cast iron works best), heat oil and add butter over medium high heat. When foam resides, add beef in as flat a layer as possible; brown but don’t cook through completely. Set aside.
Add more olive oil to the pan. Sauté onions and mushrooms until a little brown, turning down heat if needed to prevent flour from from burning to the pan. Add garlic and cook until fragrant.
Add Worcestershire sauce to deglaze. Return reserved beef to the pan and turn heat down to medium.
Stir in cream of mushroom soup and sour cream. Taste for seasoning and adjust. Serve over lo mein noodles or steamed rice.