Place the ground beef, onion, and garlic in a skillet over medium heat. Cook until the beef is browned and the onion is soft. Drain any excess fat.
Transfer the beef mixture to the slow cooker. Add the kidney beans, diced tomatoes, chili powder, cumin, and salt. Stir to combine.
Cover and cook on low for 6 to 8 hours until the chili thickens and the flavors meld.
About 30 minutes before serving, heat the oven to 400 degrees. In a bowl, stir together the cornmeal, flour, baking powder, milk, egg, and melted butter until just combined.
Pour the batter into a greased 8-inch baking dish. Bake for 15 to 20 minutes until the top is golden and a toothpick comes out clean.
Spoon the chili into bowls and place pieces of the warm cornbread on top to serve.