Cook the potatoes in a large pot of boiling water for 15 minutes or until fork tender. Drain potatoes, and then return them to the pot. (I usually cook the bacon while the potatoes are boiling.)
Add the cream, butter, sour cream, garlic powder, onion powder, salt, and pepper. Mash everything together until smooth. Check the seasoning and add more salt and pepper to taste.
Stir in 1 1/2 cups of shredded cheese, most of the bacon, and the green onions. But save some of each for topping later.
Transfer the potatoes to a 2-quart (7x11) baking dish. Spread into an even layer and top with the remaining cheese.
Bake for 20 minutes or until the cheese is melted and golden.
Remove from the oven and top with the remaining bacon and green onions. Serve warm with extra sour cream.
Notes
To make ahead, assemble the casserole and then sprinkle the cheese on top. Cover and refrigerate. Uncover and bake when ready to serve.