Cut the beef into even-sized 1-inch cubes and chop the onion into quarters, then in half.
Make the marinade for the kabobs by mixing the marinade ingredients in a large bowl or in a zip bag. Add the cubed beef, cover with plastic wrap or seal the zip bag, and marinate for 15 to 45 minutes, or up to overnight. You can also use a marinating tray to marinate the already-assembled skewers.
If using wooden skewers, soak them in water for 30 minutes while the beef marinates. This prevents them from burning during grilling.
Assemble:
Thread the cubes of meat through the skewers, alternating with the vegetables (if using). Always poke through the middle of each ingredient to prevent it from sliding or falling off. Place the assembled beef kabobs on a baking sheet for easy transfer to the grill.
Grill:
Preheat the grill to medium-high heat, about 375F, and once hot, grease your grates by oiling a folded-up paper towel on long grilling tongs.
Place the beef skewers on the grill in the opposite direction of the grill lines. Grill, for about 5 minutes per side, for a total of 10 to 15 minutes depending on the thickness, until the internal temperature of the meat reaches between 140F and 145F, to your desired doneness.
Remove the kabobs from the grill on a tray. Cover them with foil if you're still grilling other items.
Leftover storage:
Remove the beef and veggies from the skewers and refrigerate them in an airtight container for up to 4 days. To reheat, you can microwave them for about 45 seconds to 1 minute, depending how much you're heating up. To warm them up in a pan, take them out of the fridge for 10 minutes first so they're not cold. Then, drizzle a little olive oil on a non-stick pan, and heat them up over medium heat until warmed through.
Notes
Nutritional information data is for 1 (4 ounce) serving of marinated beef sirloin, after marinading, and marinade discarded.