01 - Bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Reserve 1/2 cup of the pasta water, then drain the pasta.
02 - Butterfly the chicken breasts and season both sides with salt, freshly ground pepper, and Italian herbs if desired.
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 5–6 minutes per side. Remove to a plate, cover loosely, and allow to rest.
04 - Reduce the skillet heat to medium and add unsalted butter. Sauté the chopped onion for 2–3 minutes until softened. Add minced garlic and cook for 1 minute until fragrant.
05 - Pour in the chicken broth, deglazing the skillet and scraping up any browned bits from the bottom.
06 - Stir in the heavy cream and bring the mixture to a gentle simmer. Add the grated Parmesan and stir until the sauce is smooth and creamy. Adjust seasoning with additional salt and pepper as needed.
07 - Slice the rested chicken breasts thinly. Return chicken to the skillet, then add the drained pasta. Toss to combine, incorporating a splash of reserved pasta water to achieve desired sauce consistency.
08 - Stir in chopped parsley. Serve the pasta hot, garnished with additional parsley and Parmesan as desired.