Preheat your outdoor grill to high heat, about 500°F
Place an extra-large (18" x 36") rectangular piece of heavy duty aluminum foil on your work surface.
Place the in the center of one side of the aluminium foil.
Combine the olive oil, minced garlic, oregano, black pepper and salt in a small container and whisk to combine; drizzle all over the mushrooms.
Fold the aluminum foil over the mushrooms and seal the edges by folding all open sides toward the center at least twice.
Pierce a few holes on top of the aluminium pack with a fork; place the packet on the grill pierced side up, close the lid and cook for 8 minutes, then flip so the holes are on the bottom and continue cooking for an additional 2-3 minutes.
Remove the pouch from the grill and carefully cut open the foil; garnish with chopped parsley and serve.