Preheat the oven to 450. Spread the cauliflower pieces in an even layer on a rimmed baking sheet. Drizzle with the olive oil and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Roast 15-20 minutes, until crisp-tender, then remove from the oven and leave the oven on.
While the cauliflower is roasting, mix together the sour cream, heavy cream, minced garlic, and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper.
Toss the cauliflower pieces with the sauce until coated. Mix in half of the cheddar cheese, half of the bacon, and half of the green onions.
Pour the cauliflower mixture into a casserole dish, and top with the remaining cheese and bacon. Bake 10-15 minutes, until the cheese is melty. Sprinkle with the remaining green onions. Serve hot.