01 - Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Drain and reserve 1/2 cup of the pasta water.
02 - Butterfly the chicken breasts and season both sides with salt, pepper, and dried Italian herbs if using.
03 - In a large skillet over medium-high heat, heat olive oil. Add the chicken breasts and sear until golden brown and cooked through, about 5 to 6 minutes per side. Transfer to a plate and loosely cover to rest.
04 - Reduce heat to medium. Add butter to the skillet. Sauté the 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 pan.
06 - Pour in the heavy cream and bring to a gentle simmer. Stir in Parmesan cheese until the sauce is smooth and creamy. Season with additional salt and freshly ground pepper to taste.
07 - Slice the rested chicken breasts thinly. Return sliced chicken to the pan, add the cooked pasta, and toss together. Add reserved pasta water as needed to achieve a silky sauce consistency.
08 - Stir in chopped parsley and serve immediately, garnishing with extra parsley and Parmesan cheese as desired.