Preheat the oven to 350°F (175°C). Grease a 9×9-inch baking dish.
In a large skillet over medium heat, cook the ground beef and chopped onion until the beef is browned and the onion is tender, about 7-8 minutes. Drain excess fat.
Add garlic powder, salt, and black pepper to the cooked beef mixture. Stir to combine and remove from heat.
In a large bowl, combine the Jiffy Corn Muffin Mix, sour cream, whole kernel corn, creamed corn, eggs, and melted butter. Stir until well mixed.
Spread half of the corn mixture evenly into the prepared baking dish.
Layer the cooked ground beef mixture evenly over the corn mixture in the baking dish.
Sprinkle half of the shredded cheddar cheese over the beef layer.
Spread the remaining corn mixture over the beef and cheese layer.
Top the casserole with the remaining shredded cheddar cheese.
Bake in the preheated oven for 40-45 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Remove from the oven and let cool for 5 minutes before serving.