Submitted by Dragonfly
This is what I use and it's very simple and easy. It's my own recipe and I've never measured, so I'm going to estimate the amounts I use (it's a marinade - you don't have to use exacts anyways). This recipe is for 4 large portobello mushrooms.
Portobello Mushroom Marinade (a Dragonfly original!)
3 TBS balsamic vinegar
3 TBS olive oil
1-2 tsp Worcestershire sauce (get a vegetarian one)
1-2 TBS chopped, fresh rosemary
Don't marinade for long - maybe 20 minutes, absolutely no more than 30 minutes. The mushroom will start to break down. Grill for about 3-5 minutes on each side and sprinkle with your favorite steak seasoning or salt and pepper. Toast your buns and top with your mushroom. MMMgood - oh, and a roasted red pepper sauce is excellent with it too.