Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked, shredded chicken, sour cream, cream of chicken soup, shredded cheddar cheese, milk, garlic powder, salt, and black pepper. Mix well until all ingredients are evenly combined.
Fold in the chopped green onions.
Pour the chicken mixture into a greased 9x13-inch baking dish, spreading it evenly.
In a separate bowl, mix the crushed crackers with the melted butter until well coated.
Sprinkle the buttery cracker mixture evenly over the top of the chicken casserole.
Bake in the preheated oven for 30-35 minutes, or until the casserole is bubbly and the top is golden brown.
Remove from the oven and let it cool for 5-10 minutes before serving.