Preheat oven to 375 degrees and lightly coat a 2-quart casserole dish with cooking spray.
Brown ground beef with onion in a large skillet. Once meat is cooked and onion is soft, add in garlic, chili powder, and cumin. Cook until garlic is fragrant, about 30 seconds.
In a large bowl, mix together the cornmeal, flour, sugar, baking powder, and baking soda. In a separate smaller bowl, whisk together milk, eggs and butter.