01 - Bring a large pot of salted water to a boil. Cook the fettuccine or linguine according to package directions until al dente. Drain and reserve 1/2 cup of pasta water.
02 - Butterfly the chicken breasts and season both sides with salt, pepper, and dried Italian herbs if using.
03 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and cook until golden brown and fully cooked, about 5 to 6 minutes per side. Transfer chicken to a plate, cover loosely, and let rest.
04 - In the same skillet, reduce heat to medium and add butter. Sauté chopped onion for 2 to 3 minutes until softened. Add minced garlic and cook until fragrant, about 1 minute.
05 - Pour in chicken broth, scraping up any browned bits from the skillet.
06 - Add heavy cream and bring sauce to a gentle simmer. Stir in grated Parmesan cheese until smooth and creamy. Taste and adjust seasoning with additional salt and pepper as needed.
07 - Slice cooked chicken breasts thinly. Return sliced chicken to the skillet. Add drained pasta and toss to coat in the sauce, adding a splash of reserved pasta water to adjust consistency as desired.
08 - Stir in chopped fresh parsley. Serve immediately, garnished with extra parsley and additional Parmesan cheese.