How to Make It
Preheat oven to 450°F. Melt 1 tablespoon butter in a 10-inch cast-iron skillet over medium-high. Add onion and celery; cook, stirring often, until softened, about 5 minutes. Add sage; cook, stirring constantly, 1 minute. Transfer mixture to a small bowl, and wipe skillet clean. Place skillet in oven. (Do not remove skillet from oven while preparing batter.)
Whisk together cornmeal, flour, baking powder, baking soda, sugar, poultry seasoning, salt, and pepper in a large bowl. Whisk together buttermilk and eggs in a small bowl.
Add remaining 2 tablespoons butter to hot skillet, and return to oven until butter is melted, about 1 minute. Stir onion mixture and buttermilk mixture into cornmeal mixture until just combined. Pour melted butter from hot skillet into batter, and quickly stir to incorporate. Pour batter into hot skillet, and immediately place in oven.
Bake at 450°F until golden brown and cornbread pulls away from sides of skillet, 15 to 18 minutes. Invert cornbread onto a plate; cool 5 minutes, and serve hot.