Preheat your oven to 425°F (approx. 218°C).
Place the cauliflower florets in a 3-quart baking dish. Toss with the olive oil, 1 tsp. salt, and 1⁄4 tsp. black pepper. Roast for 30 minutes.
Meanwhile, mix together 1 c. (120 g) of the shredded cheese,
the cream cheese, yogurt or sour cream, garlic powder, and the remaining 1⁄4 tsp. salt and 1⁄4 tsp. black pepper until well combined. Dollop the surface of the cauliflower with this mixture just after removing it from the oven, and carefully spread so the creamy mixture reaches all sides and corners of the pan.
Sprinkle the remaining 1 c. (120 g) shredded cheese across the surface, followed by the breadcrumbs, then drizzle the melted butter over the top.
Bake for 15–20 minutes, or until the cheese is melted and parts of the surface are slightly browned. Place under the broiler for a few seconds, if needed for more browning.
Meanwhile, cook the bacon in a skillet until browned on both sides. If it seems overly dry, spray or brush the bacon with a light coating of oil. Once cool enough to handle, chop into small pieces or strips.
When the casserole is done baking, sprinkle the surface with bacon and scallions. Serve immediately.