Brown the ground beef in a skillet over medium heat with the chopped onion and minced garlic until the meat is no longer pink. Drain any excess fat and transfer the mixture to the slow cooker.
Add the black beans, corn kernels, diced tomatoes with green chilies, chili powder, cumin, and salt to the slow cooker. Stir everything together until evenly combined.
Cover and cook on low for 5 hours.
In a mixing bowl, whisk together the cornmeal, flour, baking powder, milk, egg, and vegetable oil until a smooth batter forms.
Pour the batter evenly over the beef mixture in the slow cooker. Sprinkle the shredded cheddar cheese on top.
Cover and cook on high for 1 hour, or until a toothpick inserted in the cornbread comes out clean.