- Place beef cubes in a large sealed plastic bag. Add teriyaki marinade and seal the bag, pressing out as much air as possible. Transfer to the refrigerator and marinate for 6-8 hours.
- Thread beef onto metal skewers, alternating with the onion and colorful bell peppers. Discard the marinade.
- Combine all ingredients for the scallion sauce in a large bowl just before you begin grilling; stir occasionally and add extra soy sauce depending on your taste preference.
- Light a gas or charcoal grill. Grill the skewers over direct high heat until beef is well seared on all sides, about 3-4 minutes per side. Brush the skewers all over with additional Teriyaki Marinade & Sauce.
- Cover grill and continue to cook until center of beef is medium. Transfer skewers to platter and let it rest for 5 minutes. Serve immediately with the scallion sauce on the side for extra dipping.