Preheat your oven to 350 degrees F and spray a 9-inch x 13-inch baking dish with nonstick cooking spray. Set aside.
Melt the butter in a large skillet over medium heat. Add the onions and celery and cook for about 4 to 5 minutes until the onions are translucent. Add the herbs and cook for 1 minute.
Pour the milk into the skillet. Heat until the milk begins to bubble. Whisk in the cornmeal, sugar, salt, pepper, and nutmeg (making sure there are no lumps) and cook until thickened.
Remove the skillet from the heat and stir in the corn and baking powder. Let the mixture cool until it's only slightly warm before continuing on. (If you add the eggs while the mixture is too hot, it will scramble the eggs.)
In a medium mixing bowl, beat the egg whites with a hand mixer until soft peaks form. Set aside. Beat the egg yolks in a small bowl for 1 minute. Stir the egg yolks into the corn mixture, and then gently fold in the egg whites.
Pour the spoon bread batter into the prepared baking dish.
Bake for 30 minutes. (Don’t overbake. A toothpick inserted in the center should come out somewhat clean with a little bit of the dressing sticking to it.)
Remove the baking dish from the oven. The spoon bread may fall a bit as it cools, this is normal. Let rest for 5 minutes and then sprinkle with chopped parsley before serving warm.
Notes
If you prefer to use dried herbs instead, you'll need 1/4 teaspoon dried sage, 1/2 teaspoon dried thyme, and 1/4 teaspoon dried rosemary.
This spoon bread can also be made in a large cast iron skillet. If you use cast iron, make sure to reduce the cooking time by 5 minutes early, as the spoon bread will continue to cook in the pan once it's out of the oven.
Store any leftovers in an airtight container in the fridge for up to 5 days.