01 - Bring a large pot of salted water to a boil. Cook fettuccine or linguine according to package instructions until al dente. Drain the pasta, reserving 1/2 cup of the cooking water.
02 - Butterfly chicken breasts and season both sides with salt, freshly ground 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 cooked through, about 5 to 6 minutes per side. Transfer chicken to a plate, cover loosely, and allow to rest.
04 - Reduce skillet heat to medium and add unsalted butter. Add chopped onion and sauté for 2 to 3 minutes until softened. Stir in minced garlic and cook for an additional minute until fragrant.
05 - Deglaze the skillet with chicken broth, scraping up browned bits from the bottom. Add heavy cream and bring to a gentle simmer. Stir in Parmesan cheese until the sauce is smooth and creamy. Adjust seasoning with additional salt and pepper if needed.
06 - Slice the rested chicken breasts thinly. Return chicken to the skillet along with the cooked pasta. Toss to combine, adding reserved pasta water a little at a time to reach your desired sauce consistency. Stir in chopped parsley.
07 - Serve immediately garnished with additional parsley and extra Parmesan cheese as desired.