Brown ground beef and onion in a large skillet over medium-high heat. Drain excess fat.
Transfer beef mixture to slow cooker. Add kidney beans, crushed tomatoes, chili powder, cumin, and salt. Stir to combine.
Cover and cook on LOW for 4 hours or HIGH for 2 hours.
In a medium bowl, mix cornbread mix, egg, and milk until just combined.
Pour cornbread batter evenly over the chili in the slow cooker. Do not stir.
Cover and cook on HIGH for 1 hour, until cornbread is cooked through.
Sprinkle with cheddar cheese, cover for 5 minutes to melt, then serve hot.