Preheat the oven to 350°F. Lightly grease a 9 × 12-inch baking dish with butter or cooking spray.
Sprinkle 2 tablespoons of the cornmeal over the bottom and sides of the dish. Set the remaining cornmeal aside for the top.
Drain the whole kernel corn really well and let it sit for a few minutes so any extra liquid can run off.
Cut the jalapeño in half and finely chop it. For a milder casserole, remove the seeds and inner membranes first.
Thinly slice the scallions. Set aside a little of the green part for serving.
Whisk the eggs briefly in a large bowl. Add the sour cream and slightly cooled melted butter, then mix until smooth.
Stir in the creamed corn and drained corn.
Add the cornbread mix and fold it in just until you don’t see any dry patches.
Stir in the jalapeño, most of the scallions, and ¾ cup of the cheddar. Mix just enough to distribute everything evenly.
Add a small pinch of salt if needed, keeping in mind that the cheddar and cornbread mix may already be fairly salty.
Spread the mixture into the prepared baking dish and smooth the top.
Sprinkle the remaining ¼ cup cheddar over the casserole, followed by the remaining 2 tablespoons cornmeal.
Bake for about 45 minutes, until the edges are set and golden. The center should be cooked through but still soft and creamy.
Let it rest for 10 minutes before serving. Finish with the reserved scallion greens and serve hot or warm.