Cut each zucchini in half lengthwise; scoop out pulp, leaving a 1/4- inch-thick shell. Finely chop pulp; set pulp aside.
Steam zucchini shells, covered, 4 to 5 minutes or until crisp-tender. Place on paper towels to drain.
Heat olive oil in a large nonstick skillet over medium-high heat. Add onion, celery, and garlic; cook 3 minutes, stirring constantly, until tender. Add beef, and cook 5 minutes, stirring to crumble. Drain; set aside, and keep warm.
Add reserved zucchini pulp, tomato, and salt to skillet; cook over medium heat 15 minutes, stirring frequently. (Mixture will be dry.) Remove from heat; stir in beef mixture, basil, and oregano. Let cool slightly.
Add breadcrumbs and egg white to beef mixture; stir well.
Preheat oven to 375°.
Divide beef mixture evenly among shells (about 1/3 cup per shell). Place stuffed shells in an 11- x 7-inch baking dish coated with cooking spray. Pour pasta sauce over shells. Bake at 375° for 20 minutes or until bubbly. Remove from oven; sprinkle with cheese.