Heat oven to 375°F. Lightly grease a 9x13-inch (or 2½–3 qt) baking dish and set aside. Bring a large pot of well-salted water to a boil.
Cook the pasta until 1–2 minutes shy of al dente. Reserve 1/2 cup of the starchy cooking water, then drain.
While the pasta cooks, crisp the bacon in a large skillet over medium heat, 6–8 minutes. Transfer to a paper towel–lined plate and crumble when cool. Spoon off all but 1 tablespoon of the drippings from the skillet.
Make the sauce: Reduce heat to low and whisk in the condensed soup, sour cream, ranch seasoning, garlic powder, and 1/4 cup reserved pasta water until smooth and warm. Stir in the Parmesan and 1/2 cup of the mozzarella until just melted. Fold in the shredded chicken and half of the green onions.
Add the drained pasta to the skillet and toss to coat, loosening with another splash of pasta water if needed so the sauce clings but isn’t dry.
Transfer the mixture to the prepared baking dish. Sprinkle with the remaining 1 cup mozzarella and top with half of the crumbled bacon.
Bake uncovered for 14–18 minutes, until bubbling at the edges. For a bronzed, stretchy top, broil 1–2 minutes, watching closely.
Finish with the remaining bacon and the rest of the green onions. Let the casserole rest 5 minutes before serving so the sauce sets slightly.