Preheat oven to 450°F (232°C).
Toss cauliflower with butter, sea salt, and black pepper in a large bowl.
Roast the cauliflower until crisp-tender—about 15–20 minutes on a sheet pan or 25–30 minutes in a 1.5-quart casserole dish, stirring halfway through if needed. (Using the casserole dish saves on cleanup, while a sheet pan cooks it a bit faster.)
Whisk the sour cream and heavy cream together in a large bowl until completely smooth.
Fold in the garlic along with half of the cheddar, half of the bacon, and half of the green onions.
Give the sauce a light seasoning of salt and pepper, keeping in mind that it will become saltier as the cheese melts into it.
Pull the roasted cauliflower from the oven—keep the heat on!—then add it to the bowl and coat it in the sauce.
Transfer the mixture back into the casserole dish and finish it off with the rest of the cheese and bacon for a deliciously cheesy, crispy topping.
Return the casserole to the oven and bake 5–10 minutes, until the cheese is melted and bubbly.