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.