Preheat Oven: Set your oven to 400°F (200°C) and prepare a baking dish by greasing it lightly or spraying with non-stick cooking spray.

Prepare the Potatoes: Pierce the potatoes with a fork and bake them directly on the oven rack for about 50–60 minutes or until tender. Allow them to cool enough to handle, then peel and mash the potatoes into a smooth consistency.

Mix the Ingredients: In a large mixing bowl, combine the mashed potatoes with butter, sour cream, milk, salt, and pepper. Stir until creamy and well-blended.

Fold in the Add-Ins: Stir in 2½ cups of shredded cheese, diced bacon (save 2 tablespoons for topping), and the green onions (reserving 2 tablespoons for garnish).

Assemble the Casserole: Spoon the potato mixture into your prepared baking dish, spreading it evenly. Top with the remaining ½ cup of shredded cheese and reserved bacon.

Bake: Bake in the preheated oven for 20–25 minutes or until the cheese is melted and bubbly. If you prefer a slightly crispy top, broil for an additional 2–3 minutes.

Garnish and Serve: Once baked, remove from the oven and sprinkle the reserved green onions on top for a fresh pop of color and flavor. Serve hot.