Prep and preheat: Heat the oven to 425°F (220°C). Grease a 9x13-inch baking dish and set aside.

Roast the cauliflower: Toss florets with olive oil, salt, and pepper on a sheet pan. Roast for 18–22 minutes, stirring halfway, until lightly browned and just tender. Set aside. Lower oven temp to 375°F (190°C).

Crisp the bacon: While the cauliflower roasts, cook bacon in a skillet until crispy. Drain on paper towels, then crumble. Reserve 1 tablespoon bacon fat in the pan if you want extra flavor.

Make the sauce: In a large bowl, whisk together softened cream cheese, Greek yogurt (or sour cream), mayonnaise (if using), ranch seasoning, garlic powder, and onion powder. Stir until smooth. If the cream cheese is too firm, microwave it for 10–15 seconds to soften.

Add the cheeses: Stir in 1 1/2 cups cheddar and all of the Monterey Jack or mozzarella. Fold in half of the crumbled bacon and most of the green onions, reserving some for topping.

Combine the filling: Add cooked chicken and roasted cauliflower to the sauce. Gently fold until everything is evenly coated. Taste and adjust salt and pepper.

Assemble: Spread the mixture in the prepared baking dish. Top with the remaining 1/2 cup cheddar and the rest of the bacon.

Bake: Bake at 375°F (190°C) for 18–22 minutes, until hot and bubbly and the cheese on top is melted and lightly golden.

Finish and serve: Let the casserole rest for 5–10 minutes so it sets. Garnish with remaining green onions. Serve warm.