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 sliced chicken breasts dry and season generously with salt and freshly ground black pepper on both sides. Heat olive oil in a large skillet over medium-high heat until shimmering. Arrange the chicken in an even layer and sear for 4 to 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. Gradually whisk in the freshly grated Parmesan cheese, allowing it to melt smoothly into the sauce. If the sauce is too thick, add the reserved pasta water a splash at a time until the desired consistency is reached. Sprinkle in the dried Italian herbs if using.
05 - Add the drained pasta and seared chicken back into the skillet. Toss everything together until the pasta and chicken are evenly coated in the creamy sauce. Taste and adjust seasoning with additional salt and pepper as needed.
06 - Serve immediately in warm bowls, garnished with freshly chopped parsley and an extra shower of grated Parmesan if desired. Pair with a crisp white wine such as Pinot Grigio or Sauvignon Blanc.