Preheat oven to 375°F (190°C). Grease a 9x13 inch baking dish.
Boil the cubed potatoes in salted water until tender, about 15-20 minutes. Drain well.
Transfer the drained potatoes to a large bowl. Mash them until mostly smooth, leaving some small chunks for texture.
Stir in the melted butter, warmed milk, sour cream, half of the shredded cheddar cheese, half of the chopped green onions, salt, and pepper until well combined.
Gently fold in the cooked bacon bits.
Spread the potato mixture evenly into the prepared baking dish.
Top with the remaining shredded cheddar cheese and green onions.
Bake for 25-30 minutes, or until heated through and the cheese is melted and bubbly.
Let stand for a few minutes before serving.
Notes
This casserole is a perfect side dish for roasted meats or can be enjoyed as a hearty vegetarian main course.