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 of pasta water.
02 - While the pasta cooks, butterfly the chicken breasts and season both sides with salt, pepper, and dried Italian herbs if desired.
03 - Heat olive oil in a large skillet over medium-high heat. Add the chicken and sear until golden brown and cooked through, about 5 to 6 minutes per side. Transfer the chicken to a plate, cover loosely, and let rest.
04 - Reduce heat to medium. In the same skillet, add unsalted butter. Sauté 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.
06 - Pour in heavy cream and bring to a gentle simmer. Stir in freshly grated Parmesan cheese until the sauce is smooth and creamy. Season with additional salt and pepper if needed.
07 - Slice the rested chicken breasts thinly. Return chicken and cooked pasta to the skillet with the sauce. Toss everything together, adding a splash of reserved pasta water to adjust consistency if necessary.
08 - Stir in chopped parsley. Serve immediately, garnished with extra parsley and Parmesan cheese.