In a large skillet over medium-high heat, brown the ground beef and onion until meat is no longer pink; drain excess fat, stir in taco seasoning, pinto beans, and corn, then transfer the mixture to a greased 9x13 inch baking dish.

In a medium mixing bowl, whisk together the cornbread mix, milk, and egg until just combined, then fold in the shredded cheddar cheese.

Spread the cornbread batter evenly over the beef mixture and bake in a preheated oven at 400°F (200°C) for 18 to 20 minutes until the topping is golden brown and cooked through.