Leftovers can be frozen for convenient meals later. Let stand 5 minutes, drain well and squeeze dry. Put in large bowl. Slice and hollow out centers of tomatoes. Chop pulps finely and add to spinach.
Invert tomato shells on paper towels to drain.
Preheat oven to 350 degrees. Spoon evenly into shells. Sprinkle with remaining mozzarella and parsley. Bring 2 quarts of water to a boil, add the spinach, and cook 1 minute. Bring 4 quarts of water to a boil.