01 - Fill a large pot with salted water and bring to a rolling boil. Add fettuccine or linguine and cook according to package instructions until al dente. Drain and reserve ½ cup of pasta cooking water.
02 - Butterfly each chicken breast and season both sides generously with salt, freshly ground black pepper, and dried Italian herbs if using.
03 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and sear for 5 to 6 minutes per side, until golden and fully cooked. Remove from the skillet, cover, and allow to rest.
04 - Reduce skillet heat to medium. Melt unsalted butter, add finely chopped onion, and sauté for 2 to 3 minutes until softened. Stir in minced garlic and cook for 1 minute until fragrant.
05 - Deglaze skillet with chicken broth, scraping up any browned bits. Pour in heavy cream, bring to a gentle simmer, and stir in grated Parmesan until sauce is smooth and creamy. Taste and adjust seasoning with additional salt and pepper if needed.
06 - Thinly slice cooked chicken breasts. Return sliced chicken and drained pasta to the skillet, tossing gently to coat with the sauce. Add a splash of reserved pasta water as needed to adjust sauce consistency.
07 - Stir in chopped fresh parsley. Serve immediately, garnished with extra parsley and additional Parmesan cheese.