Heat grill up or if placing in the oven line a pan with foil (for easier clean up).
In a large 1 gallon size baggie add oyster sauce, sugar garlic, pepper flakes.Close and shake it up.
Cut bell peppers into strips and quarter the onions.add the steak tips onions and pepper strips into the baggie. Place in the fridge for 30 mins(you can leave in overnight if you would like). You can soak skewers and skewer the tips alternating with the onions and peppers or just stick them straight on the grill, your call.
cook on high heat until your desired readiness. enjoy