01 - Bring a large pot of salted water to a boil. Cook the fettuccine or linguine according to package instructions until al dente. Drain and reserve 1/2 cup of the pasta water.
02 - While the pasta cooks, butterfly the chicken breasts and season both sides with salt, pepper, and Italian herbs if desired.
03 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and sear until golden brown and cooked through, about 5 to 6 minutes per side. Transfer to a plate, cover loosely, and let rest.
04 - Reduce heat to medium in the same skillet and melt butter. Sauté chopped onion for 2 to 3 minutes until softened, then add minced garlic and cook for about 1 minute until aromatic.
05 - Deglaze the skillet with chicken broth, scraping up any browned bits from the pan.
06 - Add heavy cream to the skillet and bring to a gentle simmer. Stir in Parmesan cheese and whisk until the sauce is smooth and creamy. Season to taste with additional salt and pepper.
07 - Slice the rested chicken breasts thinly. Return chicken to the pan, along with cooked pasta. Toss to combine, adding reserved pasta water as needed to achieve your desired sauce consistency.
08 - Stir in chopped parsley, then serve immediately, garnished with extra parsley and additional Parmesan cheese.