4bunchesgreen onionswhite and light green parts only, thinly sliced
Preheat oven to 425 degrees F.
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 cauliflower florets in a 3-quart casserole dish. Toss with the reserved 2 tablespoons bacon drippings, salt, and pepper. Roast until the florets are soft and begin to brown, about 30 minutes.
Meanwhile, mix together the cream cheese and 1 cup of the Cheddar in a medium bowl until well combined. Dollop over the cauliflower, then sprinkle with the remaining 1 cup Cheddar and bake until the cauliflower is tender when poked with a knife and the Cheddar is melted and bubbly, 5 to 7 minutes more.
Dollop the sour cream evenly over the casserole and sprinkle with the scallions and reserved bacon pieces.