1. Preheat the oven to 350 degrees.
2. Cook shells according to the package directions. Drain and rinse with cold water to prevent them from cooking further. Set aside.
3. Coat 9 x 13-inch pan with cooking spray. Spread ½ cup of sauce evenly over bottom of pan and set aside.
4. For filling: in a large bowl, stir together egg whites, ricotta cheese, chicken, spinach, garlic powder and ground oregano. Stir in ¼ cup each of mozzarella cheese and parmesan cheese.
5. To assemble, spoon cheese mixture into shells and place filled shells in a single layer in baking dish. Spread remaining sauce on top. Sprinkle with remaining cheeses and bake about 30 minutes.
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