Bring a large pot of salted water to a boil. Add fettuccine and cook until al dente according to package directions. Drain, reserving 1 cup pasta water; set pasta and water aside.

Pat chicken dry and season both sides with Italian seasoning, garlic powder, salt, pepper. Heat olive oil in a large deep skillet over medium‑high heat, add chicken, and cook 5–6 minutes per side until golden and 165°F inside. Transfer to a plate, tent with foil, and rest 5 minutes; slice thinly across the grain.

In the same skillet used to cook the chicken, lower the heat to medium, melt butter, whisk in milk, then stir in salt, pepper, and garlic powder. Warm for a couple minutes, stirring occasionally.

Add cooked fettuccine and ½ cup reserved pasta water to the skillet. Toss until the liquid clings to the noodles, adding more pasta water as needed.

Turn off heat, sprinkle in Pecorino Romano and Parmesan, and stir constantly until the sauce thickens and coats the pasta, 2–3 minutes. Add sliced chicken and toss just until heated through.

Serve immediately, garnishing with extra Parmesan or parsley if desired.