Make sure you have skewers handy. You need about 6 skewers. I’d recommend using stainless steel skewers. If using bamboo skewers, soak them in water for 20 minutes before using in order to prevent them from burning.
Marinate the meat: In a large resealable bag, mix all the marinade ingredients until evenly combined. Add the cubed sirloin steak and make sure it’s all well-combined. Seal the bag, removing any excess air, and let it marinate for 3-4 hours in the fridge.
Preheat the grill over medium-high heat.
Season the veggies: In a large bowl, toss the chopped bell peppers, chopped onions, and mushrooms with Italian seasoning, garlic powder, salt (about ½ teaspoon) and a few grinds of ground black pepper.
Assemble the kabobs: assemble the skewers with the marinated beef and the seasoned veggies, alternating. I use 4-5 pieces of meat per skewer. You should have about 6 skewers. Discard the marinade.
Grill on the preheated grill for 12 minutes total (about 3 minutes per side) for medium-rare to medium. If you want them cooked more, cook for an additional minute or so per side. Tip: check the internal temperature with a thermometer, 135 F for medium-rare, 145 F for medium, and 150 F for medium-well.
Let the kabobs rest for 5-10 minutes before serving to allow the juices to redistribute. Serve and enjoy! Recommended: serve with chimichurri sauce.