01 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Reserve ½ cup of the starchy pasta water before draining. Set the drained pasta aside.
02 - While the pasta cooks, season the thinly sliced chicken breasts generously with salt and freshly ground black pepper on both sides.
03 - Heat olive oil in a large skillet over medium-high heat. Add the seasoned chicken slices in a single layer and cook for 4–5 minutes per side until deeply golden and cooked through. Transfer the chicken to a plate and tent loosely with foil to keep warm.
04 - In the same skillet, melt the butter over medium heat. Add the finely chopped onion and sauté for about 2 minutes until softened and translucent. Add the minced garlic and cook for 1 minute, stirring constantly, until fragrant.
05 - Reduce the heat to medium-low. Pour in the heavy cream and whole milk, stirring to combine. Bring the mixture to a gentle simmer—do not boil. Gradually whisk in the freshly grated Parmesan cheese, allowing it to melt smoothly into the sauce. If the sauce seems too thick, loosen it by stirring in reserved pasta water, 1 tablespoon at a time, until the desired consistency is reached. Sprinkle in the dried Italian herbs if using.
06 - Add the cooked pasta and seared chicken back into the skillet. Toss everything together until every strand or piece is evenly coated in the creamy sauce. Taste and adjust the seasoning with additional salt and pepper as needed.
07 - Divide among warmed plates or bowls immediately. Finish with a sprinkle of chopped fresh parsley and an extra dusting of Parmesan if desired. Serve alongside crusty bread and a crisp green salad.