Place the portobello mushrooms in a freezer-safe bag or other container.
In a small mixing bowl, whisk the remaining ingredients.
Pour the marinade over the mushrooms and allow to marinate for at least two hours.
Preheat your grill to medium.
Remove the mushrooms from the marinade and place, stem side up, on the grill, then allow to cook for about five minutes.
Flip and grill on the opposite side for five minutes.
Serve.