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

Melt 2 tablespoons butter in a large skillet over medium heat. Add chicken, season with salt and pepper, and cook until golden brown, about 6 to 8 minutes. Remove chicken and set aside.

In the same skillet, add chopped onion and minced garlic. Sauté for 3 to 4 minutes until onion is translucent. Add heavy cream and chicken broth, stirring to combine.

Add Italian seasoning and red pepper flakes to the skillet. Simmer for about 5 minutes, stirring occasionally, until the sauce thickens. Adjust consistency with reserved pasta water if necessary.

Return cooked chicken and drained pasta to skillet, tossing to coat in sauce. Heat for about 2 minutes to meld flavors.

Remove from heat, sprinkle with Parmesan cheese and garnish with parsley before serving.