Preheat the oven: Preheat the oven to 350°F (175°C). Lightly coat a 9 x 13-inch (23 x 33 cm) baking dish with nonstick cooking spray.

Prepare the creamy sauce: In a large mixing bowl, combine the condensed cream of chicken soup, sour cream, whole milk, garlic powder, onion powder, and black pepper. Stir until smooth and evenly blended.

Add the chicken and peas: Fold the cooked chicken and frozen peas into the sauce. Mix until everything is evenly coated. The filling should look thick and creamy rather than runny.

Add the cheddar: Fold in the shredded cheddar cheese until distributed throughout the chicken mixture.

Fill the baking dish: Transfer the chicken mixture to the prepared baking dish. Spread it into an even layer, making sure the filling reaches the corners.

Prepare the Ritz topping: Place the coarsely crushed Ritz crackers in a medium bowl. Pour the melted butter over the crumbs and toss until they are evenly coated. The mixture should resemble loose, buttery crumbs rather than a paste.

Top the casserole: Sprinkle the buttered cracker mixture evenly over the chicken filling. Do not press it down firmly, as a loose topping becomes crisper during baking.

Bake uncovered: Bake on the center rack for 35 to 40 minutes. The filling should be bubbling around the edges and the cracker topping should be golden brown.

Check the temperature: Insert an instant-read thermometer into the center of the casserole. Because the filling contains previously cooked chicken, the casserole should be reheated to at least 165°F (74°C).

Protect the topping if needed: If the crackers become deeply browned before the center reaches 165°F (74°C), cover the dish loosely with aluminum foil for the remaining baking time.

Rest the casserole: Remove the casserole from the oven and let it stand uncovered for 10 minutes. The filling will thicken slightly and become easier to scoop.

Garnish and serve: Sprinkle with chopped parsley, if using. Divide into 8 portions and serve warm.