Butter a 13-by-9-inch baking dish. In a medium saucepan, cover the wild rice with the water, add a pinch of salt and bring to a boil. Cover and simmer over low heat, stirring occasionally, until just tender, about 20 minutes. Drain the rice.
Preheat the oven to 350°. Melt 1 tablespoon of the butter in a large skillet. Add the onion and cook over moderate heat until lightly browned and just tender, about 4 minutes. Stir in the scallion and wild rice. Season lightly with salt and spread in the prepared baking dish.
In a medium saucepan, bring the milk to a simmer. Gradually whisk in the cornmeal. Whisk over moderately low heat until thickened and bubbling, about 5 minutes. Scrape the mixture into a large bowl and let stand, stirring often, until warm. Whisk in the remaining 3 tablespoons of butter, the egg yolks and Parmesan, 1 1/2 teaspoons of salt and the baking powder.
In a large bowl, beat the egg whites to firm peaks. Using a rubber spatula, stir one-quarter of the whites into the cornmeal mixture, then fold in the remaining whites until a few white streaks remain. Scrape the spoon bread over the wild rice. Bake in the center of the oven for about 30 minutes, or until puffed and golden. Let rest for 5 minutes before serving.
Make Ahead: The recipe can be prepared through Step 2 and refrigerated overnight.