Butter a 9 x 13-inch baking dish with the butter. Add the fine cornmeal to the dish and tilt the dish to coat the bottom and sides with the cornmeal. Set aside.
In a medium skillet over medium heat, cook the bacon until crisp. Transfer the bacon to the paper towels to drain. When cool enough to handle, crumble the bacon into small pieces. Set aside.
To the bacon drippings in the skillet, add the onion and celery, and cook over medium-high heat, stirring a few times, until the vegetables are softened, about 4 minutes. Increase the heat to high, add the apples, and continue to cook, stirring occasionally, until the onion is lightly caramelized and the apples softened, another 4 minutes. Remove from the heat and set aside until cooled. Stir in the pecans and bacon and season lightly with 1/4 teaspoon salt and a pinch of pepper.
Combine the milk, cream, buttermilk, and 1 1/2 teaspoons salt and 3/4 teaspoon pepper in a medium saucepan and bring just to a boil. Whisk in the stone-ground cornmeal and cook, stirring constantly, until the mixture thickens, 2 to 3 minutes. Remove from the heat and transfer the cornmeal mixture to a heatproof bowl. Stir in the cheese and set the bowl aside until the mixture is lukewarm, usually 10 to 15 minutes, stirring frequently to prevent a skin from forming.
Preheat the oven to 350°.
In a small bowl, lightly beat the egg yolks. Stir the yolks, thyme, baking soda, and baking powder into the cooled cornmeal mixture. Stir in the sautéed apple mixture.
Combine the egg whites, sugar, and cream of tartar in a large bowl and beat with an electric mixer on high speed until stiff peaks form. Working with a large rubber spatula, carefully fold one-third of the whites into the cornmeal mixture to lighten it. Add the remaining whites and gently fold in until the whites are incorporated but still fluffy.
Transfer the mixture to the prepared baking dish and bake until the spoonbread is puffed and golden brown on top and a knife inserted into the center comes out clean, 35 to 40 minutes. Serve immediately.
: Make sure you whip the egg whites to stiff peaks and then fold them in as gently as you can to keep them fluffy. You want to preserve as much of that aeration as possible to offset the density of the heavier ingredients.
This is also delicious made with pears, and you can substitute walnuts for the pecans and swap in smoked Gouda for some of the Cheddar.