Bake the cauliflower halfway: Spread the cauliflower in a single layer on a baking sheet. Dry roast in the preheated oven for 15-20 minutes or until very al dente, about ¾ of the way done. Remove and let cool slightly.
To a large bowl, add the cooled cauliflower, shredded chicken, cooked bacon, ranch dressing, green onion, minced garlic, and salt and pepper. Add most of the cheddar, reserving some to top the casserole.
Spread the cauliflower chicken mixture into a large baking dish. Bake in the preheated oven for about 15 minutes or until the cheese is melted. Broil on high for 2-3 minutes if desired for further browning.