01 - Bring a large pot of salted water to a boil. Cook the fettuccine or linguine according to package directions until al dente. Drain, reserving 1/2 cup pasta water.
02 - Butterfly the chicken breasts and season both sides generously with salt, freshly ground pepper, and optional Italian herbs.
03 - Heat olive oil in a large skillet over medium-high heat. Add the seasoned chicken breasts and sear until golden and cooked through, approximately 5 to 6 minutes per side. Transfer to a plate, cover loosely, and let rest.
04 - Reduce heat to medium and add unsalted butter to the same skillet. Sauté the chopped onion for 2 to 3 minutes until softened. Add minced garlic and cook until fragrant, about 1 minute.
05 - Deglaze the skillet with chicken broth, scraping up any browned bits. Pour in the heavy cream and bring to a gentle simmer. Stir in grated Parmesan cheese until smooth and creamy. Season with additional salt and pepper to taste.
06 - Slice the cooked chicken breasts thinly. Return chicken to the skillet along with the drained pasta. Toss to coat in the sauce, adding reserved pasta water as needed to achieve desired consistency. Fold in chopped parsley. Serve immediately, garnished with extra parsley and Parmesan as desired.