Start by combining all the ingredients for the marinade in a large bowl (olive oil, brown sugar, balsamic vinegar, crushed red pepper flakes and garlic salt) and whisk.
Cut the beef into 1-inch size cubes and place into the marinade.
Cut peppers and onion and put them in the marinade, put the cherry tomatoes and mushrooms in the marinade and stir to combine so that marinade is covering all the vegetables. Let marinate for 15 minutes, or overnight (with plastic wrap covering in the fridge if marinating overnight).
Turn searing grill to SEAR setting (or use your gas or charcoal grill!)
Take a skewer and arrange marinated beef and vegetables in alternating order on the skewer. (I like to start with beef and then do onion, pepper, tomato, beef, mushroom, pepper, onion, beef, etc.) Repeat until skewers are full and all meat and vegetables are used.
Place skewers on the searing grill and close the lid. Let grill for 5 minutes, then with tongs flip the kabob, cook for another 5 minutes. Flip, and cook for another 5 minutes.
Remove from grill and let sit rest/cool on a plate for 5 minutes before serving.