Place potatoes in a large pan, fill with water to cover potatoes, and bring to a boil. Cook until potatoes are just soft. Drain, place in a large bowl. While the potatoes are still hot, gently stir in cheese without mashing potatoes. Continue to stir, every minute or so, until cheese is melted.
In a separate bowl, stir together mayonnaise, onion, and bacon. Gently stir into potatoes and cheese. Spread into a 13x9-inch baking dish.
