Heat the oven to 400°F and choose a 10- or 12-inch ovenproof skillet. If you want crispier edges, place the empty cast-iron skillet in the heating oven briefly, then carefully remove it before adding the filling.

Set the skillet over medium-high heat and add the vegetable oil only if needed. Brown the ground beef with the chopped onion and green bell pepper for 6 to 8 minutes, breaking the meat into small pieces as it cooks; drain excess fat if necessary.

Add the minced garlic, chili powder, cumin, and salt. Stir and cook for about 30 seconds, just until the garlic is fragrant and the spices coat the beef.

Stir in the undrained diced tomatoes, drained black beans, corn, and salsa. Simmer uncovered for 3 to 5 minutes, stirring occasionally, until the mixture is hot and slightly thickened.

Stir 1/2 cup of the shredded cheese into the hot filling. Spread the mixture into an even layer so every portion will receive a balanced amount of beef, vegetables, beans, and sauce.

In a separate bowl, combine the corn muffin mix, egg, and milk. Stir only until the dry ingredients are moistened; a few small lumps are fine, and overmixing can make the cornbread dense.

Spoon and gently spread the cornbread batter evenly over the beef mixture. Sprinkle the remaining 1/2 cup cheese over the top, or reserve it for the final few minutes of baking.

Bake at 400°F for 18 to 22 minutes, until the cornbread is golden and a toothpick inserted into the topping comes out clean. If you reserved the cheese, add it during the last few minutes so it melts without becoming too dark.

Let the skillet rest for 5 minutes before serving. This short rest allows the bubbling filling to settle and makes the portions easier to scoop.