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 to 5 minutes per side until deeply golden and cooked through. Transfer to a plate and keep warm.
03 - In the same skillet, melt the butter over medium heat. Add the finely 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 - Reduce the heat to medium-low. 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, a handful at a time, until fully melted and the sauce is silky smooth. If the sauce is too thick, loosen it with small splashes of the reserved pasta water until it reaches the desired consistency. Sprinkle in the dried Italian herbs if using.
05 - Add the drained pasta and seared chicken to 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.
06 - Divide among plates and serve immediately. Garnish with freshly chopped parsley and an extra shower of grated Parmesan if desired.