DO-AHEAD TIP: Marinate beef cubes overnight or for 1 hour (see recipe). If using bamboo skewers, presoak them in water for 30 minutes, to avoid burning on the grill.
Place beef cubes in a large zipper-topped plastic bag. In a small bowl, whisk together next 5 ingredients (vinegar through garlic). Pour over beef cubes.
Seal bag and gently shake to coat well. Refrigerate overnight or for at least 1 hour. Preheat grill to MEDIUM and lightly oil grill grate.
Alternately thread beef cubes, onion quarters and bell pepper squares onto skewers, adding mushrooms to the ends of the skewers.
Grill kabobs over indirect heat for 15 to 20 minutes, turning once halfway through, until beef reaches desired level of doneness and veggies are cooked through. Watch carefully to prevent burning! Enjoy!!