Preheat & Prep: Preheat your oven to 400°F (200°C). Lightly grease a 9x13 inch baking dish.
Brown the Meat: In a large skillet, brown the ground beef with the diced onions over medium-high heat. Drain any excess grease.
Layer One: Pour the meat and veggie mixture into the bottom of your prepared baking dish. Spread it out evenly and top with 1 cup of the shredded cheddar cheese.
Mix the Topping: In a separate medium bowl, whisk together the corn muffin mix, eggs, milk, cream-style corn, sour cream, and jalapeños (if using). Don't overmix-a few lumps are okay!
Layer Two: Pour the cornbread batter over the meat and cheese. Use a spatula to spread it to the edges.
Bake: Place in the oven and bake for 30-35 minutes. About 5 minutes before it's done, sprinkle the remaining 1 cup of cheese over the top and return to the oven until the cheese is bubbly and the cornbread is golden brown.