01 - Bring a large pot of salted water to a boil. Add fettuccine or linguine and cook according to package instructions until al dente. Drain and reserve 1/2 cup of cooking water.
02 - Butterfly chicken breasts and season evenly with salt, pepper, and dried Italian herbs if desired.
03 - Heat olive oil in a large skillet over medium-high heat. Sear chicken breasts until golden brown and fully cooked, about 5 to 6 minutes per side. Transfer cooked chicken to a plate, cover loosely, and allow to rest.
04 - Reduce skillet heat to medium and add butter. Sauté onion for 2 to 3 minutes until softened. Stir in minced garlic and continue cooking until fragrant, approximately 1 minute.
05 - Pour in chicken broth, scraping up any browned bits with a wooden spoon to release flavor.
06 - Add heavy cream and bring to a gentle simmer. Fold in grated Parmesan cheese, stirring until the sauce is smooth and creamy. Adjust seasoning with salt and freshly ground pepper as needed.
07 - Slice rested chicken breasts into thin strips. Return chicken to the skillet along with the cooked pasta. Toss well to coat, adding reserved pasta water as necessary until the sauce reaches desired consistency.
08 - Stir in chopped parsley. Divide pasta among plates and garnish with additional parsley and Parmesan cheese. Serve immediately.