In a small mixing bowl, whisk together marinade ingredients.
In a large resealable plastic bag, place steak cubes, and place the mushrooms in another resealable plastic bag.
Pour 2/3 of the marinade into the steak bag, and the remaining 1/3 into the mushroom bag.
Place both bags into the refrigerator to marinate for at least 30 minutes, or up to overnight.
Preheat grill to MED heat. Skewer beef, mushrooms, red onion and green bell pepper, alternating them in whatever pattern you prefer.
Grill for 8-10 minutes, turning often, until charred and internal doneness of the steak is to your liking. You may want to use a meat thermometer to check.
Garnish with minced parsley.