Pour the marinade over the ingredients in the bag. Marinate in the fridge at least 1/2 hour and up to2 hours.At the same time, place four wooden skewers in water to soak.
Also, add the potatoes to a large saucepan and add enough water to cover by 1 inch. Bring the water to a boil and cook potatoes until just barely fork-tender, about 5-7 minutes. Drain and set aside.
Add equal amounts of steak cubes, mushrooms, potatoes, green peppers, and onions (alternating pieces) to each skewer. (2 mushrooms each and 3 each of everything else)