Cook fettuccine pasta in salted boiling water until al dente; drain and set aside.
Season and cook chicken breast in olive oil until golden and cooked through; slice and set aside.
In the same pan, sauté mushrooms, onion, and garlic in olive oil and butter until softened.
Pour in half and half; bring to a gentle simmer.
Stir in parsley, salt, and pepper, letting it thicken slightly.
Add cooked pasta and sliced chicken to the pan, tossing gently to coat.
Let everything warm together for 2–3 minutes so flavors combine.
Garnish with extra parsley and serve hot.