In a small bowl, combine the first seven ingredients; cover and chill until serving.
Spritz mushrooms with cooking spray. Using long-handled tongs, moisten a paper towel with cooking oil and lightly coat the grill rack. Grill mushrooms, covered, over medium heat or broil 4 in. from the heat for 6-8 minutes on each side or until tender. Spoon 1/2 cup tomato mixture into each mushroom cap.