Preheat oven to 400°F. Heat oil in a large oven-safe skillet over medium heat. Add diced onion and cook 4 to 5 minutes until softened. Add garlic and cook 1 minute until fragrant.
Add ground beef and cook over medium-high heat for 6 to 7 minutes, breaking it into crumbles until fully browned. Drain excess grease if needed.
Add chili powder, cumin, smoked paprika, garlic powder, salt, and pepper. Stir and cook for 1 minute to bloom the spices in the fat.
Add diced tomatoes with juices, black beans, and corn. Stir well and simmer for 5 minutes until slightly thickened. Taste and adjust seasoning.
In a medium bowl, whisk cornmeal, flour, sugar, baking powder, and salt.In another bowl, whisk egg, buttermilk, and melted butter.Combine wet and dry ingredients, mixing just until combined. Fold in 3/4 cup cheddar cheese.
Spoon cornbread batter over the filling in the skillet. Spread gently.Top with remaining cheddar cheese and jalapeño slices if using.Bake for 20 to 22 minutes until golden brown and a toothpick inserted in the center comes out clean.Let rest for 5 minutes before serving.
