Hollow out the top 1/4 inch of tomato halves with a spoon. Top each tomato half with 2 teaspoons pesto sauce and an onion slice. Arrange tomatoes in 2 foil pie pans.
Place tomatoes in the center of the grill rack. Cover and grill for 8 to 10 minutes or until tomatoes are heated through.
Stir together the cheese, almonds, and parsley in a small mixing bowl. Sprinkle mixture over tomatoes. Cover and grill for 5 minutes more or until cheese melts. Season to taste with salt and pepper, if desired. Makes 6 servings.