Preheat the oven: Set your oven to 375°F (190°C). Grease a 9x13-inch baking dish or similar casserole pan.
Cook the bacon: In a skillet over medium heat, cook the bacon until crisp. Transfer to a plate lined with paper towels and crumble when cool.
Prep the broccoli: If using fresh, steam or microwave until just tender-crisp (about 3–4 minutes).
If using frozen, thaw and pat dry. You want it dry so the casserole doesn’t get watery.
Make the sauce base: In the same skillet, add olive oil or butter. Sauté the minced garlic for 30 seconds until fragrant.
Lower the heat and add cream cheese, breaking it up with a spatula.
Whisk in liquids: Pour in the heavy cream. Stir until smooth and creamy, then mix in the ranch seasoning and red pepper flakes if using. Taste and season with salt and pepper as needed.
Combine in a bowl: In a large bowl, add the cooked chicken, broccoli, most of the bacon (reserve some for topping), cheddar, and mozzarella.
Pour the warm ranch cream sauce over and toss until everything is coated.
Assemble: Spread the mixture evenly in the prepared baking dish. Sprinkle extra cheddar on top if you like. Add the reserved bacon over the cheese.
Bake: Bake for 18–22 minutes, until the edges are bubbling and the top is lightly golden.
Finish and serve: Let it rest for 5 minutes to set.
Garnish with fresh chives or parsley. Serve warm.