3 cups milk 1 1/4 cup finely chopped scallions 3/4 cup yellow cornmeal 2 large eggs, separated 1/2 cup freshly grated Parmesan cheese 1/2 cup grated extra-sharp Cheddar cheese 1 teaspoon salt, divided 1/4 teaspoon freshly ground black pepper 5 large egg whites, at room temperature OR 10 teaspoons dried egg whites, reconstituted according to package directions
Preheat oven to 375°F. Coat a 3-quart souffle dish or similar baking dish with cooking spray.
Bring milk and scallions to a simmer in a heavy medium saucepan over medium heat. Whisking constantly, slowly sptinkle in cornmeal. Cook, whisking constantly, until mixture has thickened, about 5 minutes. Remove from heat and let cool for 10 minutes. Whisk in egg yolks, Parmesan, Cheddar, 1/2 the salt and the pepper.
About 45 minutes before serving, beat the 7 egg whites and remaining salt in a large mixing bowl with an electric mixer on high speed until soft peaks form. Whisk one-fourth of the beaten whites into the cornmeal mixture. With a rubber spatula, fold in remaining whites. Spoon batter into the prepared baking dish and smooth the top.
Bake the puff until puffed and golden brown, 30 to 35 minutes. Serve immediately.