Preheat oven to 375°F.

Place the bacon into a cold pan, put the pan on the stovetop and turn heat to medium-high. Cook the bacon, stirring occasionally, until crispy. Remove to a paper towel-lined plate.

In a medium-size bowl, whisk the heavy cream with the sour cream until smooth, set aside.

Butter a 3-quart casserole dish. Using 1/3 of the sliced potatoes, make a layer on the bottom of the casserole dish and top it with 1/3 of the bacon, 1/3 of the scallions and 1/3 of the cheese. Pour 1/3 of the heavy cream mixture over top and repeat twice, so you have three layers of potatoes.

Cover the gratin with foil. (If making ahead of time, refrigerate foil-wrapped casserole until ready to bake.) Place into the oven and cook for 1 hour. After 1 hour, remove the foil and cook for another 15 minutes or until brown and crisp on top.