01 - Bring a large pot of salted water to a boil. Cook fettuccine or linguine until al dente, following package instructions. Drain and reserve 1/2 cup of pasta water.
02 - Butterfly the chicken breasts and season both sides with salt, black pepper, and dried Italian herbs, if desired.
03 - Heat olive oil in a large skillet over medium-high heat. Add chicken and sear until golden brown and fully cooked, approximately 5–6 minutes per side. Transfer chicken to a plate and cover loosely to rest.
04 - In the same skillet, reduce heat to medium and melt butter. Sauté chopped onion for 2–3 minutes until softened, then add minced garlic and cook for about 1 minute until fragrant.
05 - Pour chicken broth into the skillet, scraping up any browned bits. Add heavy cream and bring to a gentle simmer. Stir in Parmesan cheese until the sauce is smooth and creamy. Adjust taste with salt and pepper as needed.
06 - Slice the cooked chicken breasts thinly. Return chicken and pasta to the skillet, tossing thoroughly to coat with the sauce. Add reserved pasta water gradually to adjust sauce consistency if needed.
07 - Stir in chopped parsley and serve immediately, garnished with extra fresh parsley and additional Parmesan cheese.