Preheat oven to 350° and spray a 9x13 baking dish with non-stick cooking spray. Set aside.
Add potatoes to a large pot of water and bring to a boil.
Boil the potatoes until they are tender 20-30 minutes. Drain and add to a large bowl or return back to the pan.
Using an electric mixer or hand mixer, mash potatoes, then add milk, butter, sour cream, garlic salt, and pepper, and mash until combined.
Mix in half of the shredded sharp cheddar cheese. Stir.
Spoon potato mixture into the prepared baking dish.
Top with the remaining cheese and bacon.
Bake uncovered until cheese is melted and heated through, 20-25 minutes.
Remove from oven and add sliced green onions.
Serve with your favorite main dish, salad and crusty bread, if desired.