If you're using wooden skewers, you wanna let them soak for at least 30 minutes.
To prepare the marinade for the beef, combine the balsamic vinegar, olive oil, rosemary, garlic, salt, and pepper in shallow dish. Stir until well incorporated and add beef strips. Coat strips well and try to arrange in single layer. Cover with plastic wrap and let sit in fridge for at least 30 minutes to marinate.
Prepare charcoal grill to medium high heat.
To prepare skewers, arrange about 4-5 beef strips in center and place 1 onion wedges on both ends of skewers.
The cherry tomatoes were so fragile and pretty, I didn't want them to get skewered, cook too fast, and just fall apart. So I opted to wrap them in a foil packet with 2 tablespoons of olive oil and place on indirect heat on the grill, for about 15-20 minutes, or until the tomatoes barely started to wrinkle.
Grill the skewers for about 10 minutes or so, for a good medium, or a little less if you like it more medium rare.
Toast the flatbreads on the grill for about 1-2 minutes per side, just enough to get them slightly crisp.
I like to unskewer everything and just mix it all up. That way, you can just take some tongs and throw in a random mix of ingredients!
Lightly butter inside of flatbreads, if you wish, and add kabob meat, onion wedges, and a few cherry tomatoes and you've got yourself one hell of a delicious sandwich.