01 - Melt butter in a large, deep skillet or Dutch oven over medium heat. Add diced chicken and cook until golden brown and no internal pink remains, approximately 5–6 minutes. Remove chicken to a separate plate and set aside.
02 - In the same pot, add finely chopped onion and sauté until softened, about 2–3 minutes. Stir in minced garlic and cook until fragrant, approximately 30 seconds.
03 - Pour in chicken broth, heavy cream, and whole milk. Bring the mixture to a gentle simmer.
04 - Add fettuccine to the pot, ensuring it is mostly submerged. Reduce heat to medium-low, cover, and cook for 8–10 minutes, stirring occasionally, until the pasta is nearly al dente.
05 - Return the cooked chicken to the pot. Stir in grated Parmesan cheese, salt, black pepper, nutmeg (if using), and red pepper flakes (if using). Simmer uncovered for an additional 3–5 minutes, stirring frequently, until the sauce has thickened and the pasta has reached the desired tenderness.
06 - Remove the pot from heat. Stir in chopped fresh parsley. Serve immediately, optionally garnished with additional Parmesan cheese and parsley.