Sear the Beef: Heat 1 tbsp olive oil in a large skillet over medium-high heat. Add half the beef strips (to avoid overcrowding) and cook for 1-2 minutes per side until browned but not overcooked. Remove and repeat with the remaining beef. Set aside.
Sauté Vegetables: Lower the heat to medium. Add butter to the same skillet. Once melted, add chopped onions and cook for 3-4 minutes until soft. Add garlic and mushrooms; cook until mushrooms are browned and their moisture has evaporated, about 5-6 minutes.
Deglaze and Build Sauce: Pour in beef broth, Worcestershire sauce, and Dijon mustard. Stir well, scraping up any browned bits from the pan. Let it simmer gently for 5 minutes.
Return Beef to the Pan: Add the seared beef back into the sauce. Cook for 2-3 minutes, just until heated through. Season with salt and pepper to taste.
Garnish and Serve: Sprinkle with chopped fresh parsley. Serve immediately over your choice of egg noodles, mashed potatoes, or rice.