If you already have your grill going, use it to toast the bread for this recipe, too.
For a warm appetizer, place the topped bread on the grill for 1 to 2 minutes. Cut the slices in half after heating them.
Make the mushroom mixture several hours ahead; cover and refrigerate. Toast the bread in advance, and top it off just before serving.
Add small curls of fresh Parmesan cheese and fresh marjoram leaves to the tops of these tangy treats.