Heat the oven to 375°F (190°C), then grease an 8-by-8-inch or similar 2-quart baking dish. Set the dish aside while you prepare the cauliflower and cheese sauce.

Steam or boil the cauliflower in lightly salted water for about 5–7 minutes, just until crisp-tender. Drain it very well and pat it dry so excess moisture does not thin the casserole.

Melt the butter in a saucepan over low heat. Add the cream cheese, sour cream, garlic, salt, pepper, 1/2 cup of cheddar, and the mozzarella, stirring until the mixture is smooth.

Fold the drained cauliflower into the warm cheese mixture until every floret is coated. Stir in about half of the crumbled bacon and the sliced green onions if you are using them.

Transfer the cauliflower mixture to the greased baking dish and spread it into an even layer. Sprinkle the remaining 1/2 cup cheddar over the top, followed by the remaining bacon.

Bake the casserole uncovered for 20–25 minutes, until hot and bubbling around the edges. For a more browned cheese topping, broil it for 1–2 minutes at the end while watching closely.

Let the casserole rest for 5 minutes before serving. This short rest helps the creamy sauce settle while keeping the cauliflower tender and the topping intact.