Part 1: Preparation

1. Preheat Oven and Prepare Dish: Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish or a similar 3-quart casserole dish with butter or non-stick spray.

2. Cook the Noodles: Bring a large pot of salted water to a boil. Add the egg noodles and cook according to the package directions, but undercook them by 1-2 minutes. They should be very “al dente” as they will continue to cook in the oven. Drain the noodles well and set them aside.

3. Shred the Chicken: While the noodles are cooking, remove the skin and bones from the rotisserie chicken. Shred the meat using two forks or your fingers into bite-sized pieces. You should have about 4 cups of chicken. Set aside.

Part 2: Create the Luscious Creamy Sauce

This sauce is the heart of the casserole, and it comes together in one pot on the stovetop.

1. Sauté the Vegetables: In a large skillet or Dutch oven, melt the 4 tablespoons of butter over medium heat. Add the finely chopped onion and celery. Sauté for 5-7 minutes, stirring occasionally, until the vegetables are soft, translucent, and fragrant. This step, known as “sweating” the vegetables, builds the first layer of flavor.

2. Bloom the Aromatics: Add the minced garlic and dried thyme to the skillet. Cook for 1 more minute, stirring constantly, until the garlic is fragrant. Be careful not to let it burn.

3. Make the Roux: Sprinkle the ¼ cup of all-purpose flour over the cooked vegetables. Whisk constantly for 1-2 minutes. The flour will absorb the butter and coat the vegetables, forming a paste. This is called a “roux,” and it is the classic thickener that will give your sauce a beautiful, velvety body. Cooking the flour for a minute removes any raw flour taste.

4. Build the Sauce: Slowly pour in the 2 ½ cups of chicken broth, whisking continuously to prevent lumps. Once the broth is incorporated, slowly whisk in the 1 cup of milk or half-and-half.

5. Simmer and Thicken: Bring the mixture to a gentle simmer and cook, stirring frequently, for about 3-5 minutes, until the sauce has thickened enough to coat the back of a spoon.

6. Melt in the Cheeses: Reduce the heat to low. Add the 4 oz of softened cream cheese, whisking until it has completely melted into the sauce. Turn off the heat. Add the 2 cups of shredded sharp cheddar cheese and stir until smooth and fully melted. The residual heat will be enough to melt the cheese without it becoming grainy.

7. Season: Taste the sauce and season generously with salt and freshly ground black pepper. Remember that the chicken is already seasoned, so be mindful not to over-salt.

Part 3: Assemble the Casserole

1. Combine Ingredients: To the large skillet with the finished sauce, add the drained egg noodles, the 4 cups of shredded chicken, and the 1 cup of frozen peas. Gently stir everything together until the noodles and chicken are evenly coated in the creamy sauce.

2. Transfer to Baking Dish: Pour the mixture into your prepared 9×13 inch baking dish and spread it into an even layer.

Part 4: Add the Topping and Bake to Perfection

1. Prepare the Topping: In a small bowl, combine the 1 cup of panko breadcrumbs, ¼ cup of grated Parmesan cheese, and the 3 tablespoons of melted butter. Mix with a fork until the breadcrumbs are evenly moistened.

2. Top and Bake: Sprinkle the breadcrumb mixture evenly over the top of the casserole.

3. Bake: Place the casserole in the preheated 375°F (190°C) oven. Bake, uncovered, for 20-25 minutes, or until the sauce is bubbly around the edges and the breadcrumb topping is golden brown and crispy.

4. Rest and Garnish: Remove the casserole from the oven and let it rest for 10-15 minutes before serving. This crucial step allows the casserole to set, making it easier to serve and preventing it from being runny. Garnish with fresh chopped parsley just before serving for a pop of color and freshness.