1. Preheat your oven to 375°F and lightly grease a 9×13-inch baking dish. Bring a pot of salted water to a boil for the pasta. Season the chicken breasts with salt and pepper and have all ingredients measured and ready.

2. In a large bowl, combine the softened cream cheese, sour cream, ranch seasoning mix, salt, pepper, and garlic powder, stirring until smooth and well blended. Add the raw chicken breasts to the cream cheese mixture and stir to coat evenly. Transfer the mixture to the prepared baking dish and spread into an even layer. Bake for 20 minutes.

3. While the chicken bakes, cook the shell pasta in boiling salted water until just al dente. Drain without rinsing. Remove the baking dish from the oven and, once cool enough to handle, shred the chicken directly in the dish using two forks.

4. Stir the shredded chicken and cream sauce thoroughly. Add the cooked pasta and half of the combined cheeses. Stir gently until the pasta is evenly distributed throughout the creamy mixture.

5. Sprinkle the remaining cheddar and mozzarella evenly over the top. Bake at 375°F for 5-10 minutes until the cheese begins to melt. Switch the oven to high broil and broil for 3 minutes, watching carefully, until the top is golden and slightly crispy.

6. Remove the casserole from the oven and let it rest for 2-3 minutes. Garnish with sliced green onions and serve.