Prep. Preheat your oven to 350℉ (175℃). While heating the oven, spray a 13 x 9-inch baking dish with non-stick cooking spray to prevent sticking.
Cook the pasta. Bring a large pot of salted water to a boil. Add the penne pasta and cook according to the package instructions until al dente. Drain the pasta and set it aside.
Prepare the chicken. If not using pre-cooked chicken, cook and shred 2 cups of chicken breast. A rotisserie chicken is a convenient option for this step.
Mix the base. Combine the cooked pasta, shredded chicken, Alfredo sauce, ranch seasoning, and sour cream (or ranch dressing if substituted) in a large mixing bowl. Stir until all ingredients are well combined and evenly coated.
Assemble the dish. Pour the pasta mixture into the prepared baking dish, spreading it out evenly. Sprinkle the chopped bacon over the top, followed by the shredded mozzarella cheese, making sure an even layer.
Bake the casserole. Place the baking dish in the preheated oven. Bake for 15-20 minutes until the cheese is fully melted and bubbly. The edges should be slightly golden.
Garnish and serve. Once baked, remove the dish from the oven and let it rest for 5 minutes. Garnish with the sliced green onions before serving for a fresh, crisp finish.