Add all marinade ingredients to large plastic container. Mix well.
Cut beef into 1" to 1 1/2" cubes. Toss into the marinade. Mix well, ensuring the beef is well coated.
Chop the onion, zucchini, and pepper into large chunks. Throw vegetables into large resealable bags (onion in separate bag).
Let your fire burn down to a bed of coals. Then spread coals out into a two inch thick layer.
Put your grill grate over your coals and let heat up.
Assemble your skewers, alternating each ingredient. (its helpful to put a chunk of meat on the very end to make it easy to get to so you can test doneness).
Cook skewers on grate for three to five minutes on each side.
Take a piece of beef off the end and cut into it to check if its done. If not, slide it back on and put back on the grill until done.