Heat oven to 350°. Grease a 13- by 9-inch casserole dish.

Whip egg whites in a bowl until they form peaks, then place the bowl in the refrigerator to keep cool.

In a large saucepan over medium-low heat, melt butter. Add onion and cook gently until soft. Add milk and buttermilk and bring to a simmer. Add cornmeal and sugar and cook until a thick “cornmeal mash” forms. Remove from heat, add egg yolks, and mix well. Gently fold in the whipped egg whites. Pour batter into prepared casserole dish and place in the oven. Bake until a knife inserted comes out clean, about 40 minutes. Serve warm.