Bring a large pot of salted water to a boil over high heat. Add pasta and cook according to package instructions until al dente, about 8-10 minutes. Reserve 1 cup of pasta water, then drain and set aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Season diced chicken with salt and pepper, add to skillet, and sauté for about 6-8 minutes until golden brown and cooked through. Remove chicken and set aside.

In the same skillet, add the remaining 2 tablespoons of butter. Once melted, add minced garlic and chopped onion, sautéing for about 3-4 minutes until translucent and fragrant.

Pour in heavy cream and chicken broth, stirring to combine. Add Italian seasoning and red pepper flakes, and bring to a gentle simmer. Cook for about 5 minutes until the sauce thickens slightly.

Return the cooked chicken and drained pasta to the skillet, tossing to coat thoroughly. Let it cook on low heat for an additional 2 minutes to meld flavors.

Sprinkle grated Parmesan cheese over the pasta and garnish with chopped parsley before serving.