1. Heat the oven to 375°F and lightly grease a 9-by-13-inch baking dish. Bring a large pot of salted water to a boil. Cook the elbow macaroni 1 to 2 minutes less than the package’s al dente time, then drain. Keep the pasta slightly firm because it will continue cooking in the oven.

2. Season the chicken pieces with ½ teaspoon kosher salt and ½ teaspoon black pepper. Heat the olive oil in a large skillet over medium-high heat. Add the chicken in a single layer and cook for about 6 to 8 minutes, turning as needed, until browned and the thickest pieces reach 165°F. Transfer the chicken to a plate without wiping out the skillet.

3. Reduce the heat to medium and melt 4 tablespoons of the butter in the skillet. Stir in the minced garlic, Dijon mustard, smoked paprika, crushed red pepper flakes, and cayenne. Cook for 30 to 60 seconds, stirring constantly, just until the garlic becomes fragrant. Do not let the garlic turn dark brown.

4. Stir the lemon zest, lemon juice, parsley, and chives into the butter mixture. Add the remaining 1 tablespoon butter, then sprinkle in the flour. Stir continuously for about 1 minute until no dry flour remains.

5. Gradually pour in the whole milk while whisking. Continue cooking over medium heat until the sauce thickens enough to lightly coat the back of a spoon. Reduce the heat to low before adding the cheese.

6. Add the cheddar, Monterey Jack, and Parmesan a handful at a time, stirring after each addition until melted and smooth. Do not let the sauce boil after adding the cheese. Taste before adding any additional salt.

7. Return the cooked chicken to the skillet and add the drained macaroni. Fold gently until the chicken is distributed evenly and every noodle is coated. If the mixture looks unusually thick, stir in a small splash of milk before transferring it to the baking dish.

8. Spread the chicken macaroni mixture evenly in the prepared 9-by-13-inch dish. Combine the panko with 1 tablespoon melted butter and scatter the crumbs evenly over the surface.

9. Bake at 375°F for about 15 minutes, until the sauce bubbles around the edges and the center is thoroughly hot. If the crumbs need more color, broil briefly at the end while watching continuously so the panko does not burn.

10. Remove the casserole from the oven and let it rest for 5 to 10 minutes before serving. Finish with chopped parsley if desired. The short rest allows the cheese sauce to settle while keeping the macaroni creamy.