Combine the oil, vinegar, Worcestershire sauce, mustard, and garlic in a non-aluminum dish large enough to accommodate the whole steak laid out flat.
Add the steak; turning it once to coat both sides. Cover and refrigerate for 30 minutes or overnight, turning once.
Coat a non-stick skillet with cooking spray. Place over medium-high heat.
Sprinkle the steak with the pepper and salt.
Brown for 2 minutes per side.
Add ½ cup of the broth; cook, turning once, for 5 – 6 minutes per side (for medium-rare). Remove the steak from the skillet; cover loosely with foil to keep to rest.
Reduce the heat to medium. Add the onion slices to the skillet and cook until golden brown, about 4 – 5 minutes per side. Add the remaining broth as needed to prevent the onion from sticking.
Thinly slice the steak across the grain; serve with the onions.