Preheat the oven to 350F and place a 30 centimeter cast iron pan inside to get hot.
In a large bowl, combine the flour, cornmeal, grits, baking powder, salt, sugar, jalapenos, and cheddar.
In a separate bowl, whisk the eggs, melted butter, and buttermilk.
Slowly pour the wet ingredients into the dry, and mix gently by hand.
Once your batter is ready, take the cast iron pan out of the oven and add the duck fat to the pan. Move it around so the whole surface gets coated with the fat, including the sides of the pan.
Pour the batter into the pan and return to the oven. Bake for 30 minutes, then rotate the pan and bake for another 30 minutes.
Remove the pan from the oven. Using a toothpick, poke a hole in the center of the cornbread. If the toothpick comes out clean, you’re done. If not, return the pan to the oven for another few minutes, then check again.
Let the pan cool for a few minutes, then flip it upside down onto a resting rack to cool completely.