There's no need to beat egg whites for this soufflé; it rises nicely with whisked eggs and egg whites. For overnight preparation, assemble and refrigerate the night before. Pull the baking dish out of the refrigerator as the oven preheats, and bake as instructed.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add eggplant, bell pepper, and onion; sauté 5 minutes or until tender. Stir in 1/4 teaspoon salt; sauté 5 minutes. Set aside.
Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Place flour in a medium saucepan; add milk, stirring with a whisk. Bring to a boil over medium heat, and cook milk mixture for 1 minute or until thick, stirring constantly. Add 3 tablespoons cheese, stirring until cheese melts. Remove from heat; stir in remaining 1/4 teaspoon salt and ground red pepper. Let stand for 15 minutes.
Combine eggs and egg whites in a medium bowl, stirring with a whisk. Stir in oregano. Gradually add hot milk mixture to egg mixture, stirring constantly with a whisk. Fold in eggplant mixture; pour into an 11 x 7-inch glass or ceramic baking dish coated with cooking spray. Sprinkle with the remaining 2 tablespoons cheese. Bake at 350° for 30 minutes or until puffy and set. Garnish with parsley, if desired. Serve immediately.