Preheat oven to 350°F and spray a 9x13 baking dish with cooking spray.
Boil peeled potato chunks in salted water for 15 to 20 minutes, until completely soft, then drain well. Or bake potatoes at 400°F until tender and scoop out the flesh.
Add cooked potato to a large bowl with 1 cup cheddar, sour cream, milk, melted butter, garlic, chives, salt, and pepper.
Beat with a hand mixer on medium speed for 60 to 90 seconds, until smooth and creamy.
Fold in chopped bacon with a spatula.
Spread the potato mixture evenly in the prepared baking dish and bake for 30 minutes.
Top with remaining cheddar and sliced green onions. Bake 5 more minutes, until cheese is melted.
Let sit for 5 minutes before serving.
Notes
Use a hand mixer for the creamiest texture, but stop once the potatoes are smooth so they do not turn gluey. Drain boiled potatoes well before mixing. Add the green onions only during the final 5 minutes so they stay bright. Let the casserole rest briefly for cleaner scoops.