Position a rack in the center of the oven and preheat to 350°F. Grease a 9-by-13-inch baking dish with the butter and set aside.

In a large skillet over medium heat, combine the sausage and onions, and cook, breaking up the meat frequently, until it’s cooked through and the onions have softened, 10–11 minutes. Transfer to a paper towel-lined plate to drain and set aside.

In a large bowl, whisk together the cornmeal, flour, salt, and baking soda. In a medium bowl, whisk together the eggs, buttermilk, and oil. Pour the egg mixture into the cornmeal mixture, and use a silicone spatula to fold together until just combined (the batter will still be lumpy). Fold in the reserved sausage mixture, cheese, black-eyed peas, corn, green chiles, and jalapeños. Pour the batter into the prepared baking dish, and smooth the surface with the spatula. Bake until golden brown, 50–60 minutes. Allow to cool for 10 minutes before serving.