In a glass baking dish, mix the oil, vinegar, Worcestershire sauce, mustard and garlic until well combined.
Add the steak to the baking dish. Turn to coat both sides. Cover and refrigerate 30 minutes or overnight, turning at least once.
Spray a non-stick skillet with cooking spray and place over medium-high heat.
Sprinkle steak with salt and pepper and place into heated skillet, browning for 2 minutes on each side
Add 1/2 cup of broth to the pan and continue cooking, turning once, 5-6 minutes per side or until cooked to your liking.
Remove steak from skillet and cover loosely to keep warm.
Reduce heat to medium, adding the onions to the skillet and cook until golden brown, about 3-5 minutes per side. Add remaining broth as needed to keep onions from sticking to the pan.
Serve hot with steak sliced thinly across the grain.