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 pasta water before draining. Set the drained pasta aside.
02 - While the pasta cooks, season the chicken breast slices 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 in a single layer and cook for 4–5 minutes per side until deeply golden and cooked through. Transfer 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. Add the minced garlic and cook for 1 minute, stirring constantly, until fragrant.
05 - Reduce heat to medium-low. Pour in the heavy cream and milk, stirring to combine. Bring the mixture to a gentle simmer — do not boil. Gradually whisk in the grated Parmesan cheese, allowing it to melt smoothly into the sauce. If the sauce is too thick, thin it with the reserved pasta water, adding a splash at a time. Sprinkle in the Italian herbs if using.
06 - Add the drained pasta and cooked chicken to 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.
07 - Transfer to warm serving plates or bowls immediately. Garnish with chopped fresh parsley and an extra sprinkle of Parmesan cheese if desired. Serve while hot.