Brown the ground beef in a large skillet over medium heat. Add the diced onion and minced garlic, and cook until the onion softens.
Transfer the beef mixture to the slow cooker. Stir in the kidney beans, diced tomatoes, tomato sauce, chili powder, cumin, salt, and black pepper.
Cover and cook on low for 6 hours or on high for 3 hours, stirring once halfway through if possible.
About 30 minutes before the chili finishes, heat the oven to 400 degrees. In a mixing bowl, combine the cornmeal, flour, baking powder, milk, egg, and vegetable oil. Stir until just mixed.
Pour the batter into a greased 8x8 baking pan. Bake for 20 minutes until the top is golden and a toothpick comes out clean.
Let the cornbread cool for a few minutes, then cut into squares. Serve the chili in bowls with a piece of cornbread on the side.