Preheat the oven to 425 degrees and spray a 3-quart casserole dish with nonstick spray. Set aside. Cook the bacon in a medium nonstick skillet over medium heat, stirring occasionally, until brown and crispy, 6 to 7 minutes. Transfer to a paper towel-lined plate. Reserve 2 tablespoons of the bacon drippings. Put the potatoes into the prepared casserole dish. Toss with the reserved 3 tablespoons bacon drippings, 1 teaspoon salt, and 1/2 teaspoon pepper. Roast until the potatoes are soft and begin to brown, about 45 minutes. Then sprinkle shredded Cheddar and bake until the cheese is melted and bubbly and the potatoes are completely cooked through when poked with a knife, 13 to 15 minutes. Dollop the sour cream evenly over the casserole and sprinkle with the scallions and bacon pieces.