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. Sear the chicken 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 - In the same skillet, reduce heat to medium and 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 additional minute until fragrant, being careful not to let it brown.
04 - Lower 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 allow it to boil. Gradually whisk in the grated Parmesan cheese, a handful at a time, until fully melted and the sauce is velvety smooth. If the sauce seems too thick, loosen it with splashes of the reserved pasta water until the desired consistency is reached. Sprinkle in the dried Italian herbs if using.
05 - Return the seared chicken to the skillet and add the drained pasta. 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.
06 - Divide among warmed plates or shallow bowls. Garnish with freshly chopped parsley and an extra shower of grated Parmesan. Serve immediately while the sauce is still silky and warm.