01 - Bring a large pot of salted water to a boil. Cook fettuccine or linguine according to package instructions until al dente. Drain, reserving 1/2 cup of the pasta cooking water.
02 - While the pasta is cooking, butterfly both chicken breasts and season generously with salt, freshly ground pepper, and Italian herbs if desired.
03 - In a large skillet over medium-high heat, warm olive oil. Add the prepared chicken and sear until golden and cooked through, about 5 to 6 minutes per side. Remove chicken from the skillet, tent with foil, and let rest.
04 - Reduce heat to medium. In the same skillet, melt the unsalted butter and sauté the finely chopped onion for 2 to 3 minutes, until softened. Stir in minced garlic and cook until fragrant, about 1 minute.
05 - Deglaze the skillet with chicken broth, scraping up any caramelized bits. Pour in heavy cream and bring to a gentle simmer. Stir in freshly grated Parmesan cheese until the sauce is smooth and creamy. Season to taste with additional salt and pepper.
06 - Slice rested chicken breasts thinly. Return sliced chicken to the skillet along with cooked pasta. Toss thoroughly, adding reserved pasta water gradually until the desired sauce consistency is achieved.
07 - Stir in chopped parsley. Plate immediately, garnishing each portion with extra parsley and additional Parmesan cheese as desired.