01 - Bring a large pot of salted water to a boil. Cook fettuccine or linguine following package instructions until al dente. Drain, reserving 1/2 cup of the pasta water.
02 - Butterfly chicken breasts and season both sides evenly with salt, black pepper, and Italian herbs if using.
03 - In a large skillet over medium-high heat, add olive oil. Sear seasoned chicken breasts for 5 to 6 minutes per side until golden and fully cooked. Transfer chicken to a plate, cover loosely, and let rest.
04 - Reduce heat to medium. Add butter to the same skillet. Once melted, sauté chopped onion for 2 to 3 minutes until softened. Add minced garlic and sauté for 1 additional minute until fragrant.
05 - Deglaze the skillet with chicken broth, scraping up any browned bits from the bottom.
06 - Stir in heavy cream and bring the sauce to a gentle simmer. Add grated Parmesan cheese and stir until the sauce is smooth and creamy. Adjust seasoning with additional salt and pepper as needed.
07 - Slice rested chicken breasts thinly. Return sliced chicken to the skillet with the sauce. Add cooked pasta and toss gently to combine, adding reserved pasta water as necessary to achieve desired sauce consistency.
08 - Stir chopped parsley into the skillet and mix thoroughly. Serve immediately, garnished with extra parsley and a sprinkle of Parmesan cheese.