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. Arrange the chicken slices in a single layer and cook without moving for 4–5 minutes until a deep golden crust forms. Flip and cook another 4–5 minutes until cooked through. Transfer to a plate and tent loosely with foil to keep warm.
04 - Reduce the heat to medium and melt the butter in the same skillet. Add the chopped onion and sauté for about 2 minutes until softened and translucent. Stir in the minced garlic and cook for 1 minute until fragrant, being careful not to let it brown.
05 - Pour in the heavy cream and whole milk, stirring to combine. Bring the mixture to a gentle simmer — do not let it boil. Gradually whisk in the grated Parmesan cheese, a handful at a time, until completely melted and the sauce is velvety smooth. If the sauce seems too thick, loosen it with splashes of the reserved pasta water until it reaches the desired consistency. Sprinkle in the Italian herbs if using.
06 - Add the drained pasta and seared chicken back into the skillet. Toss everything together, coating the pasta and chicken evenly in the creamy sauce. Taste and adjust the seasoning with additional salt and pepper as needed.
07 - Serve immediately in warm bowls, garnished with freshly chopped parsley and an extra sprinkle of Parmesan cheese if desired.