Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with cooking spray or butter to prevent sticking.

In a large mixing bowl, combine the diced cooked chicken, mayonnaise, sour cream, and cream of chicken soup, stirring until all ingredients are well incorporated and the chicken is evenly coated.

Add the diced celery, sliced almonds, and shredded cheddar cheese to the chicken mixture, folding everything together until the ingredients are distributed throughout.

Season the mixture with salt, black pepper, and garlic powder, then stir again to ensure the seasonings are evenly dispersed.

Transfer the chicken mixture to your prepared baking dish, spreading it out evenly with a spatula to create a uniform layer that will bake consistently.

In a separate bowl, combine the crushed butter crackers with the melted butter, tossing with a fork until all the cracker crumbs are coated with butter.

Sprinkle the buttered cracker crumbs evenly over the top of the chicken mixture, creating a complete covering that will become the golden, crispy topping.

Place the baking dish in the preheated oven and bake for 30-35 minutes, until the casserole is bubbling around the edges and the cracker topping has turned golden brown.

Remove from the oven and let the casserole rest for 5 minutes before serving to allow the filling to set slightly, making it easier to serve neat portions.

Serve hot directly from the baking dish, scooping generous portions onto plates and enjoying this comforting, creamy casserole while it's fresh from the oven.