Follow these detailed steps to ensure your chicken macaroni casserole is a flawless success every time. The key is in the technique, particularly when making the cream sauce.

Step 1: Prepare the Oven and Baking Dish
Preheat your oven to 375°F (190°C). Generously grease a 9×13 inch (or a similar 3-quart) baking dish with butter or cooking spray. This crucial step prevents the casserole from sticking and makes cleanup significantly easier.

Step 2: Cook the Macaroni
Bring a large pot of heavily salted water to a rolling boil. Add the elbow macaroni and cook according to the package directions, but be sure to undercook it by about 2 minutes. You are aiming for a very firm “al dente” texture. The pasta will finish cooking in the oven, and this step prevents it from becoming mushy and waterlogged in the final dish. Once cooked, drain the macaroni well and set it aside. Do not rinse the pasta.

Step 3: Begin the Creamy Béchamel Sauce
In a large, heavy-bottomed pot or Dutch oven, melt the 1/2 cup of unsalted butter over medium heat. Once the butter is fully melted and sizzling slightly, add the finely diced yellow onion. Sauté the onion for 4-5 minutes, stirring occasionally, until it becomes soft, translucent, and fragrant. Be careful not to let it brown.

Step 4: Create the Roux
Sprinkle the all-purpose flour over the cooked onions and melted butter. Whisk constantly for about 60 to 90 seconds. This process, known as making a “roux,” cooks out the raw flour taste and is the foundation for a thick and stable sauce. The mixture should be bubbly and look like wet sand.

Step 5: Gradually Add the Milk
While continuing to whisk constantly, begin adding the warmed milk to the roux. It’s critical to add the milk very slowly at first—about a half-cup at a time. Whisk vigorously after each addition to ensure the mixture is completely smooth before adding more. This gradual process is the secret to a lump-free sauce. Once all the milk has been incorporated, continue to cook the sauce over medium heat, stirring frequently, for about 5-8 minutes, until it has thickened enough to coat the back of a spoon.

Step 6: Incorporate the Cheese and Seasonings
Remove the pot from the heat. This is an important step to prevent the cheese from separating or becoming grainy. Immediately begin adding the 4 cups of freshly grated sharp cheddar cheese, one handful at a time, whisking until each addition is completely melted and smooth before adding the next. Once all the cheese is incorporated, stir in the salt, black pepper, garlic powder, and smoked paprika. Taste the sauce and adjust the seasonings if necessary. It should be well-seasoned, as the pasta and chicken will absorb a lot of the flavor.

Step 7: Combine All Components
To the pot with the finished cheese sauce, add the drained macaroni, the shredded cooked chicken, the diced ham, and the frozen peas. Use a large spatula or wooden spoon to gently fold everything together until every piece of pasta and chicken is generously coated in the glorious cheese sauce.

Step 8: Assemble the Casserole
Pour the macaroni mixture evenly into your prepared 9×13 inch baking dish. Use the spatula to spread it into an even layer.

Step 9: Prepare and Add the Topping
In a small bowl, combine the Panko breadcrumbs with the 2 tablespoons of melted butter. Use a fork to toss them together until the breadcrumbs are evenly moistened. Sprinkle this mixture evenly over the top of the casserole. This will create that coveted crispy, golden-brown crust.

Step 10: Bake to Perfection
Place the baking dish on a baking sheet to catch any potential drips. Bake, uncovered, in the preheated 375°F (190°C) oven for 25-30 minutes. The casserole is done when the sauce is bubbling vigorously around the edges and the Panko topping is a deep golden brown.

Step 11: Rest and Serve
Remove the casserole from the oven and let it rest on a wire rack for at least 10-15 minutes before serving. This resting period is crucial; it allows the sauce to set up and thicken slightly, ensuring that you can serve neat, cohesive portions instead of a soupy mess. Garnish with fresh parsley if desired, and serve warm.