Wash and dry the mushrooms. Grab the stem between your finger and thumb and gently twist to remove. Set aside.
Whisk together all of the rest of the ingredients and pour into a shallow baking dish. Put the mushrooms in the dish and use a pastry brush to coat them with the marinade. Allow the mushrooms to marinate for 5-15 minutes and then flip them over and continue for another 5-15 minutes.
Meanwhile, heat the grill to medium high. Oil the grates.
Cook the mushrooms caps on the grill until charred and cooked, about 5 minutes per side.