Many thanks to my dear friend Joanna who served these beautiful and oh-so-flavorful mushroom "steaks" for dinner the other night. I thought I had died and gone to Vegetarian Heaven! We had them as the main course, but they can also be served as an appetizer or on a sandwich/burger. Joanna made her own pesto with fresh basil, toasted pine nuts and extra virgin olive oil. Prepared pesto will work fine, but if you can, I suggest making your own for the best flavor. These are super easy and you can make them ahead of time and refrigerate till ready to grill. Serving size indicates number of mushrooms. As an appetizer, one mushroom per serving will do, but for a main course I would recommend 4 servings of 2 mushrooms each. The recipe originated in Vegetarian Times and I'm posting here for safekeeping.