Prepare the Potatoes:
Preheat your oven to 375°F (190°C).
Place the cubed potatoes in a large pot, cover with water, and add a pinch of salt. Bring to a boil and cook until the potatoes are fork-tender (about 15-20 minutes).
Mash the Potatoes:
Drain the potatoes and transfer them to a large mixing bowl.
Add sour cream, milk, melted butter, and 1 cup of shredded cheddar cheese. Mash or mix until smooth and creamy. Season with salt and pepper to taste.
Assemble the Casserole:
Spread the mashed potato mixture evenly into a 9x13-inch baking dish.
Sprinkle the remaining cheddar cheese and crumbled bacon over the top.
Bake the Casserole:
Cover the dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Garnish and Serve:
Remove from the oven and let the casserole cool slightly.
Garnish with sliced green onions and serve warm.