Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package instructions until al dente (typically 8-10 minutes). Reserve 1 cup of pasta water, drain and set aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add the diced chicken pieces and season with salt and pepper. Cook for about 6-8 minutes until the chicken is golden brown and cooked through, stirring occasionally. Remove the chicken from the skillet and set aside.

In the same skillet, add the remaining butter and sauté the minced garlic and diced onion until translucent (about 3-4 minutes).

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 about 5 minutes until it thickens slightly. Adjust the consistency with reserved pasta water if needed.

Return the cooked chicken and drained pasta to the skillet with the sauce. Adjust seasoning with additional salt and pepper if needed. Toss everything together to ensure the chicken and pasta are coated, and let cook for another 2 minutes to meld the flavors.

Sprinkle grated Parmesan cheese over the top and let it melt into the dish. Garnish with fresh parsley before serving.