01 - Bring a large pot of salted water to a boil. Cook the fettuccine or linguine according to package instructions until al dente. Drain well, reserving ½ cup of the cooking water.
02 - While the pasta cooks, butterfly the chicken breasts and season both sides with salt, freshly ground black pepper, and Italian herbs if desired.
03 - Heat olive oil in a large skillet over medium-high heat. Add the seasoned chicken breasts and sear until golden brown and cooked through, about 5 to 6 minutes per side. Transfer to a plate, cover loosely, and allow to rest.
04 - Reduce skillet heat to medium. Add unsalted butter and sauté the finely chopped onion for 2 to 3 minutes until softened. Incorporate the minced garlic and sauté until aromatic, about 1 minute.
05 - Deglaze the pan with chicken broth, scraping up any browned bits. Pour in the heavy cream and bring to a gentle simmer. Stir in freshly grated Parmesan cheese until the sauce is smooth and creamy. Adjust seasoning with additional salt and pepper as needed.
06 - Thinly slice the rested chicken breasts. Return chicken and cooked pasta to the pan with the sauce. Toss together, adding reserved pasta water a splash at a time to reach desired sauce consistency.
07 - Stir in chopped parsley and serve immediately, garnished with extra parsley and additional Parmesan if desired.