Preheat the oven to 400 degrees F.
Measure out the butter, sour cream, and milk and allow the ingredients to reach room temperature while the potatoes are cooking.
Scrub the potatoes clean. Dry them and rub with olive oil. Place them on a baking sheet and bake for 40 minutes, or until tender. You can also cook the potatoes in the Instant Pot.
While the potatoes bake, cook the bacon in a skillet over medium heat. Flip the strips over periodically until evenly cooked. Remove them to a plate with a paper towel to soak up the grease.
Remove the potatoes from the oven. Lower the heat to 350 degrees F.
Leave the skins on two of the potatoes. Peel the rest.
In a large bowl, crush the potatoes. Crumble the bacon and add 3/4 of it to the bowl. Add the butter, sour cream, milk, 1/2 the grated cheese, and salt and pepper to the bowl.
Stir until well combined. Don’t over crush the potatoes. Mixture should still contain some pieces of potato.
Add the potatoes to a greased baking dish. Top the casserole with the reserved shredded cheese.
Bake, uncovered, for 20 minutes. Top with sliced reserved bacon and green onions for garnish.