Preheat oven to 350°.
Combine first 6 ingredients and 1/4 teaspoon salt in a small bowl. Set aside.
Combine fat-free ricotta, black pepper, remaining 1/4 teaspoon salt, eggs, and egg whites in a medium bowl.
Heat nonstick skillet over medium-high heat. Coat pan with cooking spray. Add onion; sauté 2 minutes. Add mushrooms; sauté 2 minutes. Add spinach; sauté 1 minute. Stir in egg mixture. Wrap handle of pan with foil. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Top with tomato mixture.