How to Make It
Preheat oven to 400°F. Prepare cornbread batter according to package instructions. Spread batter evenly in a greased 12-inch ovenproof skillet. Bake until a toothpick inserted in center comes out clean, 10 to 12 minutes.
In a large skillet over medium-high heat, cook ground beef, seasoning with salt and pepper and breaking up any large chunks, until lightly browned and cooked through, approximately 5 minutes. Transfer meat to a bowl; drain fat from skillet, leaving 1 Tbsp. behind. Heat fat over medium heat and add onion and mushrooms. Season with salt and pepper. Cook, stirring occasionally, until mushrooms are golden brown and onion is soft, approximately 6 minutes.
Spread sauce over cornbread to cover. Arrange onion-mushroom mixture and ground beef evenly over sauce. Top with cheese, then bake until cheese has melted, 3 to 4 minutes. Slice into 8 wedges and serve immediately.