01 - Bring a large pot of salted water to a boil. Cook the fettuccine or linguine according to package directions until al dente. Drain the pasta, reserving 1/2 cup of the cooking water.
02 - Butterfly the chicken breasts and season both sides with salt, freshly ground pepper, and dried Italian herbs if using.
03 - Heat olive oil in a large skillet over medium-high heat. Add the chicken breasts and sear until golden brown and fully cooked, about 5 to 6 minutes per side. Transfer the chicken to a plate, cover loosely, and allow to rest.
04 - In the same skillet, reduce heat to medium. Add unsalted butter and 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 from the bottom. Pour in the heavy cream and bring to a gentle simmer. Stir in freshly grated Parmesan cheese until smooth and creamy. Adjust seasoning with additional salt and pepper as needed.
06 - Slice the chicken breasts thinly. Return the sliced chicken and cooked pasta to the pan. Toss to coat in the sauce, adding reserved pasta water as needed to achieve desired consistency.
07 - Stir in chopped parsley and serve immediately, garnishing with additional parsley and extra Parmesan cheese if desired.