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 - Pat the chicken breast slices dry and season both sides generously with salt and freshly ground black pepper. Heat olive oil in a large skillet over medium-high heat until shimmering. Sear the chicken in a single layer for 4–5 minutes per side until deeply golden and cooked through. Transfer to a plate and tent loosely with foil to keep warm.
03 - Reduce the heat to medium. In the same skillet, melt the butter. 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 grated Parmesan cheese a handful at a time, allowing each addition to melt smoothly before adding more. If the sauce is too thick, loosen it with small splashes of the reserved pasta water until it reaches a silky, coat-the-back-of-a-spoon consistency. Sprinkle in the dried Italian herbs if using.
05 - Add the drained pasta and seared chicken to the skillet. Toss everything together over medium-low heat until the pasta is evenly coated and the chicken is warmed through. Taste and adjust the seasoning with additional salt and pepper as needed.
06 - Divide among warmed plates or shallow bowls. Finish with a sprinkle of chopped fresh parsley and an extra dusting of Parmesan if desired. Serve immediately while the sauce is still velvety and hot.