Special equipment: a 9-inch well-seasoned cast-iron skillet
Preheat the oven to 350 degrees F. Pour the vegetable oil in a 9-inch cast-iron skillet that has been well seasoned to prevent sticking.
Combine the cornbread mix, onion, cheese, broccoli, black pepper and jalapeno in a large bowl. Stir in the sour cream, buttermilk and melted butter to make a thick batter.
Heat the oil in the skillet over medium heat until hot and shimmering. Spoon in the batter, spreading it to the edges. The oil will come up around the edges of the batter. Spread it over the top of the batter. (During the baking process the oil will help brown the bread.)
Bake until golden brown and a toothpick inserted in the center comes out clean, about 45 minutes. Cool in the pan for 10 minutes, and then carefully loosen the edges and bottom with an egg turner or large spatula and turn onto a rack. Serve warm or at room temperature.