Heat a large skillet over medium heat and cook the ground beef with the diced onion and minced garlic 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, black beans, diced tomatoes, chili powder, cumin, salt, and black pepper. Stir to combine.
Cover and cook on low for 6 hours or on high for 3 hours, until the flavors have blended and the chili has thickened.
About 30 minutes before the chili is done, preheat the oven to 400°F and grease an 8-inch baking dish.
In a mixing bowl, whisk together the cornmeal, flour, and baking powder. In a separate bowl, mix the milk, egg, vegetable oil, and honey. Pour the wet ingredients into the dry ingredients and stir until just combined.
Spread the cornbread batter into the prepared dish and bake for 20 minutes, or until a toothpick inserted in the center comes out clean.
Serve the chili in bowls with slices of warm cornbread on the side.