01 - Bring a large pot of generously 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 on both sides with salt and freshly ground black pepper. 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 the chicken to a plate and keep warm.
03 - In the same skillet, melt the butter over medium heat. 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 - 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 grated Parmesan cheese, allowing each addition to melt smoothly before adding more. If the sauce is too thick, thin it with the reserved pasta water, a splash at a time, until it reaches a silky, coating consistency. Sprinkle in the dried Italian herbs if using.
05 - Return the seared chicken to the skillet along with 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 - Serve immediately in warmed bowls. Garnish with freshly chopped parsley and an extra shower of grated Parmesan if desired.