Cook beef, onion, and garlic in a large saucepan coated with cooking spray over medium-high heat until browned, stirring frequently to crumble the beef. Drain the beef mixture in a colander.
Return beef mixture to pan. Add chili powder, cumin, sugar, oregano, tomato sauce, pinto beans, and chiles; cover and cook over medium-low heat for 10 minutes. Pour mixture into a 2-quart casserole coated with cooking spray; set aside.
Combine the milk and egg in a bowl, and stir well. Add the cornmeal mix, and stir well. Pour the cornmeal mixture over the beef mixture. Bake at 400° for 20 minutes or until the corn-bread topping is lightly browned.
Make-Ahead Tips: You can assemble the casserole ahead of time, omitting the corn-bread topping, cover and chill in the refrigerator or freeze (thaw frozen casserole overnight in the refrigerator).