1 large red onion, sliced into 1 1/2-inch cubes (about 3 onion layers each)
Whisk together oil, Worcestershire, soy sauce, lemon juice, mustard, garlic, brown sugar, and black pepper in a small bowl. Place beef cubes and mushrooms in a large resealable plastic bag. Add marinade and seal bag, pressing out as much air as possible. Marinate, refrigerated, for at least 1 and up to 5 hours.
Thread beef onto skewers, alternating with mushrooms and onions.
Light one chimney full of charcoal. When all charcoal is lit and covered with gray ash, pour out and spread the coals evenly over entire surface of coal grate. Set cooking grate in place, cover grill and allow to preheat for 5 minutes. Clean and oil the grilling grate. Grill skewers until beef is well seared on all sides and center of beef registers between 125-130°F on an instant read thermometer, about 3 minutes per side. Transfer skewers to platter and let rest for 5 minutes. Serve immediately.