Stir oil, salt and pepper together in a large bowl. Add cauliflower slices and gently toss to coat. Transfer to a large rimmed baking sheet and roast until tender, 10 to 14 minutes. Top with cheese, return to the oven and roast until the cheese has melted, 1 to 2 minutes. Top each cauliflower slice with a dollop of sour cream and sprinkle with bacon and scallion greens.