1. Wash whole zucchini and parboil for 10 minutes. Drain and cool. Cut lengthwise. Scoop out the pulp and chop it finely. Set aside.
2. Saute garlic, onion, green peppers, celery, and mushrooms until tender. Add marjoram, parsley, and zucchini pulp. Heat. Remove from heat and cool.
3. Fold in stiffly beaten egg whites. Fill the cavity of each zucchini half with the mixture.
4. Place stuffed zucchini in a shallow nonstick baking pan. Bake at 350° for 30 minutes.