Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the shredded cooked chicken, frozen peas, and frozen carrots.
In a separate bowl, whisk together the condensed cream of chicken soup, sour cream, mayonnaise, milk, garlic powder, onion powder, salt, and black pepper until smooth.
Pour the soup mixture over the chicken and vegetables and stir until everything is evenly coated.
Transfer the mixture to a 9×13 inch baking dish and spread it out evenly.
In a small bowl, mix the crushed Ritz crackers with the melted butter until the crackers are evenly coated.
Sprinkle the buttered cracker mixture evenly over the top of the casserole.
Bake in the preheated oven for 35 to 40 minutes, or until the casserole is bubbly and the cracker topping is golden brown.
Remove from the oven and let it cool for 5 minutes before serving.