Combine garlic, balsamic vinegar, Dijon mustard, Worcestershire sauce, Italian seasoning, salt, black pepper, and crushed red pepper flakes in a large Ziplock bag.
Add steaks and seal the bag, removing as much air as possible.
Marinate steaks in the fridge for 1 hour or overnight. Pro tip: place your Ziplock bag in a bowl inside the fridge. If it leaks, you’ll avoid the mess.
About thirty minutes before you plan on cooking your London broil steaks, remove them from the fridge. Allow the steaks to come to room temperature while still in the Ziplock bag.
Preheat your Z Grills pellet smoker to 500°F.
Once your grill and steaks are ready, lightly grease the grill grates. You can use cooking spray or a paper towel soaked in vegetable oil.
Place steaks directly on the grill grate at a 45° angle. Close the lid and grill for 2-3 minutes. Then, rotate the steak 90° to form the crosshatch pattern of the London broil. Grill for 2-3 minutes more.
Flip the steaks and repeat the previous step for the other side.
Once a meat thermometer reads 145° when inserted in the center of the thickest part of the steak, remove your grilled London broil from the heat. This will yield a medium steak.
Allow your grilled London Broil steaks to rest for 5-10 minutes.