Preheat the oven to 325 degrees F (165 degrees C).

Cook and stir beef and onion in a large skillet over medium-high heat until beef is browned and crumbly, 5 to 7 minutes; drain and discard grease.

Stir ground beef mixture, sour cream, corn, mushroom soup, chicken soup, and egg noodles together in a large bowl; season with salt and black pepper. Transfer to a 9x13-inch baking dish.

Bake in the preheated oven until egg noodles are tender, about 45 minutes. Sprinkle Cheddar cheese over top of casserole; continue baking until cheese is melted, 10 to 15 minutes more.