01 - Bring a large pot of salted water to a boil. Add the pasta and cook until al dente according to package instructions. Drain well and reserve 1/2 cup of the pasta cooking water.
02 - Butterfly the chicken breasts. 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 chicken breasts and sear until golden brown on both sides and cooked through, approximately 5 to 6 minutes per side. Transfer chicken to a plate, cover loosely, and let rest.
04 - Reduce skillet heat to medium. Melt unsalted butter, then add chopped onion and sauté for 2 to 3 minutes until softened. Stir in minced garlic and cook for about 1 minute until aromatic.
05 - Pour in chicken broth, scraping the pan to release browned bits. Allow the liquid to simmer for 1 minute.
06 - Add heavy cream and bring to a gentle simmer. Stir in grated Parmesan cheese until the sauce becomes smooth and creamy. Adjust salt and pepper as needed.
07 - Slice the cooked chicken breasts thinly. Return chicken to the skillet along with cooked pasta. Toss to coat, adding reserved pasta water gradually until the desired sauce consistency is reached.
08 - Stir in chopped fresh parsley. Plate and garnish with additional parsley and extra Parmesan cheese if desired. Serve immediately.