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. Heat olive oil in a large skillet over medium-high heat. Sear the chicken for 4–5 minutes per side until deeply golden and cooked through. Transfer to a plate and keep warm.
03 - Reduce the skillet heat to medium. Melt the butter in the same pan, scraping up any browned bits from the chicken. 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.
04 - 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 a handful at a time, allowing each addition to melt smoothly before adding more. If the sauce is too thick, thin it with splashes of the reserved pasta water until it coats the back of a spoon. Sprinkle in the dried Italian herbs if using.
05 - Add the drained pasta and seared chicken to the skillet, tossing everything together so the sauce clings to every strand or piece. Taste and adjust the seasoning with additional salt and pepper as needed.
06 - Transfer to warm serving plates or a large platter. Finish with a generous sprinkle of chopped fresh parsley and pass extra Parmesan at the table. Serve immediately while the sauce is still velvety and hot.