Remove stems from mushrooms; reserve for another use.
Grill mushrooms, bell peppers, and tomatoes, covered with grill lid, over medium-high heat (350° to 400°) 10 minutes or until peppers and tomatoes look blistered and mushrooms are blackened, turning occasionally.
Place bell peppers in a heavy-duty zip-top plastic bag; seal and let stand 10 minutes to loosen skins. Peel peppers; remove and discard seeds. Peel tomatoes. Chop peppers, tomatoes, and mushrooms.
Stir together grilled vegetables, green onions, and next 8 ingredients. Cover and chill 1 hour. Garnish, if desired.