Bring a large pot of salted water to a boil. Add pasta and cook until al dente, about 8-10 minutes. Reserve 1 cup of pasta water, then drain the pasta.

In a large skillet, melt butter over medium heat. Add diced chicken, season with salt and pepper, and cook for 6-8 minutes until golden brown. Remove and set aside.

In the same skillet, add remaining butter, garlic, and onion. Sauté for 3-4 minutes until onion is translucent.

Pour in heavy cream and chicken broth; stir. Add Italian seasoning and red pepper flakes. Simmer for 5 minutes until slightly thickened.

Return chicken and pasta to the skillet, toss to combine. Add reserved pasta water if sauce is too thick.

Serve with freshly grated Parmesan cheese and garnish with parsley.