In a large pot or Dutch oven over medium-high heat, cook the ground beef and chopped onion. Break the meat apart with a spoon until it is browned and the onion is translucent. Drain any excess fat. Add the minced garlic and cook for one more minute until fragrant.
Stir in the chili powder and cumin, coating the meat evenly. Add the diced tomatoes, kidney beans, and beef broth. Bring the mixture to a boil, then reduce the heat to low. Cover and let it simmer for 20 to 25 minutes, stirring occasionally to let the flavors meld.
While the chili simmers, preheat your oven to 400°F (200°C) and grease an 8-inch square baking pan. In a large bowl, whisk together the cornmeal, flour, sugar, baking powder, and salt. In a separate small bowl, beat the egg and stir in the milk and melted butter. Pour the wet ingredients into the dry ingredients and stir until just combined.
Pour the batter into the prepared baking pan. Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown. Let it cool for a few minutes before slicing into squares.
Ladle the hot beef chili into bowls. Serve each portion with a fresh square of warm cornbread on the side or crumbled right on top of the chili.