Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.

Bring a large pot of salted water to a boil. Cook the pasta according to package directions, but remove it about 2 minutes early so it is still al dente. Drain well and set aside. Do not rinse the pasta.

While the pasta cooks, cook the bacon in a large skillet over medium heat until crispy. Transfer to a paper towel-lined plate to drain, then crumble into bite-sized pieces. Reserve some for garnish.

In a large bowl, combine the softened cream cheese, sour cream, milk, and the packet of dry ranch seasoning. Whisk until the mixture is smooth and creamy. Season with salt and pepper, being mindful of the salt from the bacon and cheese.

To the bowl with the ranch sauce, add the cooked pasta, shredded chicken, about three-quarters of the crumbled bacon, and 1 cup of the shredded cheddar cheese. Stir gently until everything is evenly coated.

Pour the mixture into the prepared baking dish and spread it into an even layer.

Sprinkle the remaining 1 cup of cheddar cheese and all of the mozzarella cheese evenly over the top of the casserole.

Bake, uncovered, for 20-25 minutes, or until the casserole is hot and bubbly and the cheese on top is melted and lightly golden. For a crispier top, you can place it under the broiler for 1-2 minutes at the end, watching it closely to prevent burning.

Remove from the oven and let the casserole rest for 5-10 minutes. This allows the sauce to set. Garnish with the remaining crumbled bacon and the sliced green onions before serving.