Preheat the broiler or fire up your grill. Or, if you’re me, put a grill pan on your stove and coat it with an equal mixture of Pam and your own tears, because you don’t own a grill and your oven is so old that the broiler is actually a drawer underneath the stove that licks tongues of hungry flames upon your food as soon as you open it, charring it beyond recognition and causing a cringing Pavlovian response every time someone says "broil."
Place the mushrooms in a large, shallow dish (either a large baking dish or rimmed baking sheet will work well) Combine the olive oil, balsamic vinegar, lemon juice, garlic, and parsley in a bowl and mix well. Pour over mushrooms and season with salt and pepper. Marinate for 30 minutes, turning once.
Place the mushrooms on a grill grate (or aforementioned grill pan) or broiler pan (gill side down if grilling, gill side up if broiling) and cook for 4-6 minutes per side. The mushrooms should give off some of their natural liquid and be mostly firm to the touch. Serve hot.
Recipe by Baked In at https://www.baked-in.com/2011/12/18/portabella-steaks/