Place potatoes into a large saucepan, cover with water, and bring to a boil. Reduce heat to medium-low and simmer potatoes until tender but not mushy, about 15 minutes. Drain and let cool. Cut potatoes into 1-inch thick slices.
Whisk cream of chicken soup, sour cream, 1/2 cup melted butter, and green onions in a bowl until thoroughly combined. Whisk cheese into the sauce. Mix corn flake crumbs with 1/4 cup melted butter in a separate bowl.
Spread a thin layer of the sour cream sauce into the bottom of a 9×13-inch glass baking pan. Spread a layer of potato slices over sauce, followed by a thick layer of sauce. Continue layers, ending with sauce. Sprinkle corn flakes mixture over the top of the casserole.
Bake in the preheated oven until casserole is bubbling and the corn flakes topping is crisp, about 45 minutes.