Prep the oven and dish: Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with butter or oil.
Cook the bacon: Crisp the bacon in a skillet over medium heat.
Drain on paper towels and crumble. Reserve 1 tablespoon of bacon fat if you want extra flavor for the broccoli.
Par-cook the broccoli: Steam or microwave the florets until crisp-tender, about 3–4 minutes. If using the skillet, sauté them in the reserved bacon fat for 3–5 minutes.
Season lightly with salt and pepper. They should still have some bite.
Make the ranch sauce: In a mixing bowl, whisk together softened cream cheese, sour cream, and heavy cream until smooth. Stir in ranch seasoning, garlic, 1 cup shredded cheddar, and 1/2 cup mozzarella.
Taste and adjust salt and pepper.
Combine the filling: In a large bowl, add the cooked chicken, broccoli, most of the crumbled bacon (save some for topping), and green onions if using. Pour the ranch cheese sauce over and fold until evenly coated.
Assemble: Spread the mixture in the prepared baking dish. Top with the remaining cheddar and mozzarella.
Sprinkle the reserved bacon on top.
Bake: Bake uncovered for 18–22 minutes, or until the casserole is hot, the cheese is melted, and the edges are bubbling. For extra color, broil for 1–2 minutes at the end.
Rest and serve: Let it rest for 5–10 minutes so it sets slightly. Garnish with extra green onions or a light sprinkle of ranch seasoning.
Serve warm.