Preheat the oven to 425 degrees F (220 degrees C). Grease a 10-inch oven-proof skillet.

Cook sausage and onion in a separate skillet over medium heat until sausage is browned and crumbly and onion is translucent, 5 to 7 minutes; drain well.

Mix corn, cornmeal mix, sour cream, milk, oil, and eggs together in a large bowl; pour 1/2 of the mixture into the prepared skillet. Sprinkle with sausage mixture and Cheddar, then pour remaining cornmeal mixture over top.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 40 minutes. Let stand 10 minutes before serving.