Preheat and prepare: Heat your oven to 375°F (190°C).
Grease a 9x13-inch baking dish with olive oil or butter.
Cook the bacon: In a skillet over medium heat, cook bacon until crisp. Transfer to a paper towel-lined plate and crumble. Reserve 1 tablespoon of bacon fat.
Sauté aromatics: In the same skillet, melt the butter with the reserved bacon fat.
Add onion and cook 3-4 minutes until soft. Stir in garlic for 30 seconds until fragrant.
Make the creamy ranch sauce: Reduce heat to low. Add cream cheese to the skillet and stir until smooth.
Whisk in heavy cream and ranch seasoning. Simmer 2-3 minutes until slightly thickened. Taste and season with salt and pepper.
Prep the veggies: Lightly steam broccoli and cauliflower until just tender-crisp (about 3-4 minutes).
Drain well so the casserole doesn’t get watery.
Combine in the baking dish: Add cooked chicken, broccoli, cauliflower, and half the crumbled bacon to the dish. Pour the ranch sauce over everything and toss gently to coat.
Add cheese: Sprinkle cheddar and mozzarella evenly over the top. Finish with the remaining bacon.
Bake: Bake 18-22 minutes, until the casserole is bubbling and the cheese is melted and lightly golden.
Rest and garnish: Let it sit 5 minutes to set.
Sprinkle with chopped chives or parsley before serving.