Add the balsamic vinegar, olive oil, garlic, dijon, salt, thyme, pepper, and rosemary to a gallon-sized ziplock bag. Massage the bag to mix the marinade ingredients together.
Add the steak cubes to the bag, seal the bag, and massage the marinade into the steak. Place the bag in the fridge for at least 2 hours or up to 8 hours to marinate.
Prepare the grill and preheat it over high heat.
Prep the veggies by coring and deseeding the bell peppers and peeling the onion. Cut the bell pepper and onion into 1-inch pieces, and cut the zucchini into 1/2-inch thick slices. Then pour out the steak and marinade into a bowl. (The trick is to try and make the veggies and steak about the same size so everything touches the grill evenly and gets charred.)
To assemble the kabobs, add the steak and veggies to the skewers. I like to alternate colors for a pretty presentation - steak, zucchini, bell peppers, onion, and repeat ending with a piece of steak. You can assemble them however you like.
Once you have 8 skewers place them on a baking sheet and brush with the remaining marinade, focusing on the veggies.
Add the kabobs to the grill and cook for 10-15 minutes, depending on your desired temperature. Flip them every so often so they char and cook evenly.
Take the kabobs off the heat when a large piece of the steak reaches an internal temperature of 130 to 135 degrees F for medium-rare. Let rest for about 10 minutes before serving.
Notes
If you prefer using wooden skewers, soak them in water for 30 minutes before you're ready to assemble and grill. This will help prevent the skewers from burning up while the food cooks.
You can also use NY strip, sirloin, or flat iron steak for this recipe.
If you prefer to use fresh herbs you'll need 3 teaspoons of fresh thyme and 1 teaspoon of fresh rosemary, finely minced.
Keep leftovers in an airtight container in the fridge for up to 3 days or freeze for up to 3 months.
To reheat, I find it easiest to place the kabobs on a baking sheet under a very hot broiler and turn them around a few times until warm.