Cook unopened spinach on high for 4 minutes. Let stand 5 minutes, drain well and squeeze dry. Put in large bowl. Slice and hollow out centers of tomatoes. Discard seeds. Chop pulp finely and add to spinach. Invert tomatoes shells on paper towels to drain. Preheat oven to 350 degrees.
Add 1/2 cup mozzarella cheese, onion, parmesan, salt & pepper to spinach mixture and blend well. Spoon evenly into shells. Sprinkle with remaining mozzarella and parsely. Arrange in 8-inch round glass or ceramic baking dish & cook at 350 degrees for 6 minutes or until heated through.