Preheat the oven to 350°F. Spray an 8x8-inch baking dish with non-stick cooking spray.
Place cauliflower in a medium pot and cover with 1-inch of water. Bring to a boil and cook for 3 minutes, until slightly tender. Drain well and return to the pot.
Use a potato masher to mash the cauliflower to a firm, chunky consistency, just enough to break up the larger pieces.
Add the cream cheese and sour cream, stirring to fully coat the cauliflower. Stir 1 cup cheese, 1/4 cup scallions, salt, and pepper. Adjust seasoning to taste.
Transfer to the prepared baking dish. Bake for 35 minutes, until the edges are bubbly. Garnish with the remaining 1 tablespoon of scallions. The sauce will thicken slightly as it stands. Serve warm.