Boil the cubed potatoes in a large pot of salted water until tender, about 15-20 minutes. Drain and set aside.
In a large mixing bowl, combine the shredded chicken, sour cream, cheddar cheese, mozzarella cheese, milk, green onions, garlic powder, onion powder, salt, pepper, and smoked paprika. Mix until well combined.
Add the drained potatoes to the chicken mixture and gently fold them in, being careful not to mash the potatoes too much.
Transfer the mixture to a greased 9x13 inch baking dish, spreading it out evenly.
If using, sprinkle the crispy fried onions evenly on top of the casserole for added crunch.
Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the casserole is bubbling and the top is golden brown.
Once done, allow to cool for about 5 minutes before serving. Garnish with additional chopped green onions.