Bring a large pot of salted water to a rolling boil. Cook the pasta according to package instructions until al dente, about 8 to 10 minutes. Reserve 1 cup of pasta water before draining. Set pasta aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add the chicken pieces, season with salt and pepper, and cook for 6 to 8 minutes, stirring occasionally, until golden brown and cooked through to an internal temperature of 165 degrees F. Remove and set aside.

In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Add the minced garlic and diced onion. Saute for 3 to 4 minutes until soft and fragrant.

Pour in the heavy cream and chicken broth, stirring to combine. Add the Italian seasoning and red pepper flakes. Let the sauce simmer gently for 5 minutes until slightly thickened. Add reserved pasta water a splash at a time if the sauce becomes too thick.

Return the cooked chicken and drained pasta to the skillet. Toss everything together until fully coated in the sauce. Cook for 2 more minutes to meld the flavors. Taste and adjust seasoning with salt and pepper as needed.

Sprinkle the grated Parmesan over the top and let it melt into the sauce. Garnish with fresh parsley and serve warm.