- Preheat the oven to 400 degrees F.
- Put the bacon in a 12-inch cast-iron skillet; set over medium heat and cook, stirring occasionally, until the bacon is browned and crisp, 7 to 8 minutes. Transfer to a paper towel-lined plate, leaving the fat in the skillet.
- Add the diced onion to the hot skillet and cook, stirring, until translucent, about 4 minutes. Add the garlic and cook 1 more minute. Transfer the onion and garlic to a separate plate and set aside. Keep the skillet over low heat.
- Whisk together all ingredients for the cornbread with jalapenos in another large bowl; add the cheese, onion, and garlic and whisk mix with a wooden spoon to combine.
- Brush the bottom and side of the skillet with the fat left in it (add a pat or two of butter if there’s not enough). Carefully add the batter (it should sizzle) and sprinkle the bacon evenly on top.
- Transfer the skillet to the oven and bake until the top is deep golden brown and a toothpick inserted into the center comes out clean, 20 to 25 minutes. Let cool 10-15 minutes before serving.