Bring a large pot of salted water to a boil. Cook the pasta one minute less than the package directions for perfect texture. Drain and set aside.
In a large saucepan, whisk together milk and softened cream cheese over medium heat until smooth. Stir in the ranch seasoning, then season with salt and pepper to taste. Heat until slightly thickened and bubbly.
In a large bowl, mix cooked chicken with warm ranch sauce. Fold in sautéed onions if using. Add cooked pasta and toss until evenly coated.
Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish and spread the chicken pasta mixture evenly. Top with half the shredded cheese and sprinkle bacon crumbles on top.
Add the remaining cheese and bake uncovered for 20–25 minutes or until golden and bubbly.
Let rest for 5–10 minutes before serving. Garnish with chopped parsley or green onions for a fresh finish.
