Preheat your oven to 425°F. Line a rimmed baking sheet with parchment paper.
Arrange the cauliflower florets on the baking sheet. Liberally spray them with olive oil and season them with kosher salt and black pepper. Roast the cauliflower until tender-crisp, for about 12 minutes.
Meanwhile, in a large bowl, mix the sour cream, milk, Dijon, garlic, cayenne, and ½ cup cheddar.
When the cauliflower is done, add it to the sour cream mixture and mix to combine.
Transfer the mixture to a greased 1.5-quart baking dish. Top with the remaining shredded cheddar. Return the baking dish to the oven and bake just until the cheese is melted, for about 10 minutes.
Sprinkle the casserole with the bacon bits and scallions and serve.
