Wash and scrub the red potatoes. Place in a large pot an cover with water. Bring to a boil and cook until the potatoes are cooked through by piercing with a fork or wooden skewer. Carefully pour the water off of the potatoes. Cool completely, Remove the skins of the cooled potatoes. Coarsely shred the potatoes or cut into bite-size pieces. Set aside.
In a medium-saucepan mix and heat the butter, soup, sour cream, onion, and salt. Cook until the butter is melted and the sauce is smooth. Remove from the heat and stir in the shredded cheese. The cheese does not have to be completely melted.
Stir 2/3 of the mixture into the cooled and shredded potatoes. Gently stir to thoroughly coat the potatoes. Pour the potato mixture into a buttered 9 x 13-inch baking dish. Spread with remaining sauce on top then sprinkle with cornflake mixture.