01 - Bring a large pot of salted water to a boil. Cook fettuccine or penne according to package directions until al dente. Reserve ½ cup of pasta water before draining. Set the drained pasta aside.
02 - Season the thinly 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 keep warm.
04 - Reduce the heat to medium. Melt the butter in the same skillet. Add the finely chopped onion and sauté for 2 minutes until softened. Stir in the minced garlic and cook for 1 additional minute until fragrant.
05 - 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 until fully melted and the sauce is smooth and velvety. If the sauce is too thick, add the reserved pasta water one tablespoon at a time until the desired consistency is reached. 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 sauce. Taste and adjust seasoning with additional salt and pepper as needed.
07 - Divide among serving plates immediately. Garnish with freshly chopped parsley and an extra sprinkle of Parmesan cheese if desired.