Clean mushrooms and remove stems. Simmer mushroom caps in salted water for 5 minutes or until mushrooms are tender. Drain and transfer to a bowl. Add olive oil, vinegar, garlic, red onion, oregano, parsley, salt, peppercorns and coriander. Stir to combine. Taste and adjust seasoning. Marinate in the refrigerator for several hours or overnight. Bring to room temperature before serving.