01 - Fill a large pot with water, salt generously, and bring to a boil. Add fettuccine or linguine and cook until al dente following package instructions. Drain pasta and reserve 1/2 cup of cooking 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 and sear for 5 to 6 minutes per side until golden and fully cooked. Transfer chicken to a plate, cover loosely, and allow to rest.
04 - In the same skillet, reduce heat to medium and melt butter. Add chopped onion and cook for 2 to 3 minutes until softened. Stir in minced garlic and cook for 1 minute until fragrant.
05 - Pour in chicken broth, scraping up browned bits from the pan. Stir in heavy cream and bring to a gentle simmer. Sprinkle in grated Parmesan cheese and mix until the sauce is smooth and creamy. Season with additional salt and pepper as needed.
06 - Slice rested chicken thinly. Return sliced chicken to the skillet along with cooked pasta. Toss to combine thoroughly, using reserved pasta water as needed to adjust the sauce consistency.
07 - Fold in chopped parsley, then serve immediately, garnished with extra parsley and Parmesan cheese as desired.