Spoonbread is often served as a side dish, but this version takes center stage as a main course. It is heartier than a souffle but light enough for a satisfying lunch or supper, especially on a cool day. If there are any leftovers, dice them and add to a green salad.
Preheat oven to 400 degrees F. Grease an 8-cup baking dish.
In a large saucepan, combine stock, milk, salt, and sage. Bring just to a boil over medium-high heat. Gradually whisk in cornmeal. Cook, stirring constantly for 1 minute. Reduce heat and cook for 2 to 3 minutes, until thickened. Remove from heat. Stir in butter, egg yolks, turkey, corn, cheddar, scallions, and baking powder.
In a bowl, beat egg whites until medium firm peaks form. Stir ¼ of the beaten egg whites into turkey mixture, then fold in remaining whites. Gently spoon mixture into the prepared baking dish. Bake for 30 minutes or until puffed and slightly jiggly in the center. Serve immediately.