Preheat the oven to 400ºF.
Lay bacon on an aluminum foil-lined sheet pan and bake 20-25 minutes or to preferred crispiness. Remove from the oven, cool, and crumble.
Add cubed potatoes to a pot of boiling water. Boil 10 minutes, drain, and set aside.
In a medium bowl, combine sour cream, mayonnaise, milk, ranch seasoning, salt, pepper, and half of the shredded cheese. Whisk until blended well, then gently fold in drained potatoes.
Pour in a baking dish, spread evenly, and sprinkle on remaining shredded cheese.
Bake 20 minutes then broil on high 1 minute. Remove from the oven and garnish with bacon and green onion.