Preheat the oven: Preheat the oven to 375°F and lightly grease a 9x13-inch baking dish.
Make the creamy base: In a large bowl, stir together the softened cream cheese, sour cream, ranch seasoning, milk, garlic powder, and pepper until smooth.
Add the chicken, cheese, and bacon: Fold in the shredded chicken, 1 cup of the cheddar, and about two-thirds of the crumbled bacon until everything is evenly coated.
Spread and top: Spread the mixture into the baking dish. Sprinkle the remaining 1 cup cheddar and the rest of the bacon over the top.
Bake: Bake uncovered for 20-25 minutes, until the casserole is hot and bubbling and the cheese is melted and just starting to brown at the edges.
Garnish and serve: Sprinkle with sliced green onions and let it rest for a few minutes before serving warm.
Notes
Soften the cream cheese fully so the sauce stays smooth. This is the ranch-and-bacon version; keep it distinct from million dollar chicken casserole, which uses a buttery cracker topping instead. Since the chicken is already cooked, bake just until the casserole is hot throughout (165°F) and bubbling.