Heat grill, or grill pan, to medium-high heat (350 to 450 degrees F).
Wash and dry mushrooms. Slice off any woody stems/ends.
Add the olive oil, soy sauce, Worcestershire sauce, mustard, vinegar, garlic powder, and black pepper to a mixing bowl. Stir until fully combined.
Toss the mushrooms in the olive oil mixture and allow to marinate for 3 to 4 minutes.
Thread the mushrooms onto wooden or metal skewers.
Place the mushroom skewers on the grill and allow to grill for 2 to 3 minutes till grill lines have formed. Flip and allow to grill for another 2 to 3 minutes till they reach the desired doneness.
Remove mushrooms from the grill and skewers. Toss with optional chopped parsley. Serve immediately.