Preheat oven to 425 °F. Line a rimmed baking sheet with parchment-paper.
Organize the cauliflower-florets on the baking sheet. Liberally spray olive-oil & season with kosher-salt and black-pepper. Roast the cauliflower till tender-crisp, about 12 mins.
In a big bowl, combine the milk, sour-cream, dijon, cayenne, garlic and ½ cup cheddar.
When cauliflower is done, add it to the sour-cream sauce & blend to combine.
Transfer the mixture to a greased 1.5 quart baking dish. Top with the remaining shredded-cheddar. Return to the oven & bake just until cheese is melted, about 10 mins.
Sprinkle the cauliflower-casserole together with the bacon pieces & also the scallions and serve.