Cook onion in butter over medium heat until tender.
If using fresh mushrooms, add them to the skillet with garlic powder and cook until liquid is released. Add cream cheese and stir until creamy.
Whisk in beef broth and Worcestershire, simmer until creamy and slightly thick, about 3 minutes.
Add the beef to the skillet and cook an additional 3-4 minutes or until heated through. Remove from heat and stir in sour cream.
Notes
Check the deli area of the grocery store for fully cooked beef roast, or use leftover pot roast or roast beef.
Alternately, use any leftover cooked beef (including ground beef) for this recipe.
To cook steak:
slice steak into ½-inch pieces
heat oil over medium-high heat and cook steak in small batches until brown (1-2 minutes per side). It does not have to be cooked through, it will finish cooking in the sauce
remove from skillet and proceed with the recipe above