Preheat oven to 400°F. Spray an 8x8 casserole dish with non-stick cooking spray and set aside.
Saute ground beef, onion, and bell pepper in a skillet over medium heat until the ground beef is browned and the onion is soft and translucent. Drain the excess fat/liquid.
To the ground beef mixture, add garlic powder, yellow mustard, ketchup, brown sugar, and salt and pepper to taste, along with 1⁄2 cup water. Mix thoroughly.
Bring the mixture to a boil, reduce heat and cover. Simmer for 10-15 minutes.
Transfer cooked sloppy joe mixture to the prepared casserole dish.
Whisk together cornbread mix, egg, and milk until combined. Spoon over the top of the sloppy joe mixture and carefully spread to cover the mixture.
Bake for 15-20 minutes until the cornbread is golden brown.
Video
Notes
Store, covered, in the refrigerator for 3-4 days or freeze up to 3 months.
For a thicker cornbread layer, you could use two packages of cornbread mix - cook time may need to be adjusted. Use a toothpick to check for doneness.
Double the recipe for a 13x9 baking dish. You may need to add extra time in the oven if doing so. Use the toothpick method to check for doneness.