1. Set up grill to medium heat and allow to preheat while preparing the mushrooms.
2. Whisk together in a bowl the lemon juice, parsley, olive oil, and garlic.
3. Lay the portabello mushrooms out in a glass baking dish and drizzle the mixture over, coating all mushrooms well. Use your fingers to move the mushrooms around to ensure they are coated all over.
4. Let sit at room temperature for 10 minutes.
5. Put mushrooms on grill grate, cover grill, and cook for 5 minutes, carefully turn, and cook for 5 minutes longer, just until you have nice grill marks and the mushrooms are tender. Yields four servings.
This can be served as a sandwich or on a salad.