01 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Before draining, reserve ½ cup of the starchy pasta water. Drain the pasta and set aside.
02 - While the pasta cooks, season the sliced chicken breasts generously with salt and freshly ground black pepper on both sides.
03 - Heat olive oil in a large skillet over medium-high heat. Add the seasoned chicken slices in a single layer and cook for 4 to 5 minutes per side until deeply golden and cooked through. Transfer the chicken to a plate and tent loosely with foil to keep warm.
04 - 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 additional minute until fragrant, being careful not to let it brown.
05 - Reduce the heat to medium-low. Pour in the heavy cream and whole milk, stirring to combine. Bring the mixture to a gentle simmer, then gradually whisk in the grated Parmesan cheese until fully melted and the sauce is silky smooth. If the sauce is too thick, thin it out by stirring in the reserved pasta water a splash at a time. Sprinkle in the dried Italian herbs if using.
06 - 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 the seasoning with additional salt and pepper as needed.
07 - Divide among serving bowls immediately while hot. Garnish with freshly chopped parsley and an extra shower of grated Parmesan if desired. Serve alongside crusty bread and a crisp green salad.