Preheat the oven to 375°F. Grease a 9x13-inch baking dish.
Cook the pasta in salted water until just al dente. Drain and set aside.
In a large skillet, melt butter over medium heat.
Add garlic and cook for 30 seconds, stirring often.
Stir in heavy cream and milk or chicken broth. Simmer gently for 2–3 minutes.
Reduce heat to low and stir in parmesan, Italian seasoning, salt, black pepper, and red pepper flakes if using.
Add cooked pasta and chicken to the sauce. Stir until evenly coated.
Transfer the mixture to the prepared baking dish.
Sprinkle mozzarella evenly over the top.
Bake for 18–22 minutes, or until the cheese is melted and the sauce is bubbling.
Broil for 1–2 minutes if you want a more golden top.
Rest for 5 minutes, garnish with parsley if desired, and serve warm.