Preheat oven to 425°F.
Place bacon in a large nonstick skillet over medium heat; cook until crisp, 6 to 8 minutes. Transfer to a paper-towel-lined plate and let cool. (Reserve the drippings in the pan.)
Combine cauliflower, pepper, salt and the bacon drippings in a 9-by-13-inch baking dish. Roast, stirring twice, until tender, about 35 minutes.
Meanwhile, combine 1 cup cheese, ⅔ cup sour cream and half the scallions in a small bowl. When the cauliflower is tender, stir the cheese mixture into the cauliflower in the pan. Sprinkle with the remaining ¼ cup cheese. Bake until hot, 5 to 7 minutes more.
Chop the cooled bacon. Sprinkle the hot casserole with the bacon and the remaining scallions.
