01 -
Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch (23x33 cm) baking dish.
02 -
Cook pasta in a large pot of salted boiling water until just al dente. Drain and set aside.
03 -
In a large skillet, melt 2 tablespoons butter over medium heat. Add onion and cook for 3 minutes, until softened. Add garlic and cook for 1 minute more, stirring constantly.
04 -
Sprinkle flour over the onion and garlic. Stir and cook for 1 minute.
05 -
Gradually whisk in the milk, ensuring no lumps. Cook, whisking, until thickened, about 3–5 minutes.
06 -
Remove from heat. Stir in Parmesan, mozzarella, salt, black pepper, and red pepper flakes (if using) until cheese is melted and sauce is smooth.
07 -
Add cooked pasta and chicken to the sauce. Stir in chopped parsley. Mix until well combined.
08 -
Pour mixture into the prepared baking dish.
09 -
In a small bowl, combine breadcrumbs with melted butter. Sprinkle evenly over the pasta. Top with extra Parmesan if desired.
10 -
Bake for 20–25 minutes, or until the top is golden and the sauce is bubbling.
11 -
Garnish with more chopped parsley before serving.