Preheat oven to 350°.
Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1 cup. Set aside.
Cut each zucchini in half lengthwise; scoop out pulp, leaving a 1/4-inch-thick shell. Set pulp aside. Steam zucchini shells, covered, 6 minutes or until tender. Drain, cut sides down, on several layers of heavy-duty paper towels.
Place zucchini pulp in food processor; process until finely chopped. Spoon into a bowl. Place spinach, basil, garlic, and onion in food processor; process until finely chopped.
Heat olive oil in a large nonstick skillet over medium heat. Add spinach mixture; cook 3 minutes, stirring occasionally. Add zucchini pulp, tomato, and salt; cook 8 minutes, stirring occasionally. Remove from heat; stir in breadcrumbs.
Fill each zucchini half with about 2 1/2 tablespoons zucchini mixture. Sprinkle evenly with cheese. Bake at 350° for 20 minutes or until cheese melts. Garnish with parsley, if desired.