- Whisk together oil, Worcestershire, soy sauce, mustard, garlic and brown sugar in a small bowl.
- Place beef cubes in the bowl.
- Thread beef onto skewers, alternating with mushrooms and peppers.
- When all charcoal is lit and covered with gray ash – 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.