1½ pounds beef sirloin tips, cut into 1 ½-inch cubes
1large red onion, cut into bite-size pieces
1large red bell pepper, cut into bite-size pieces
1large yellow bell pepper, cut into bite-size pieces
1large orange bell pepper, cut into bite-size pieces
Whisk together oil, Worcestershire, soy sauce, lemon juice, mustard, garlic, brown sugar, and black pepper in a small bowl. Place beef cubes in a large resealable plastic bag. Add marinade and seal the bag, pressing out as much air as possible. Marinate, refrigerated, for at least an hour or up to 4 hours.
Remove the meat from the fridge and discard the marinade. Thread meat, onion, and peppers onto the skewers, alternating meat and vegetables. There should be about 2 to 3 pieces of meat per skewer.
Cook kabobs on the grill over medium-high, about 6-8 minutes each (turning to make sure they cook evenly). Serve warm and enjoy!