Cook the pasta: Bring a large pot of salted water to a boil. Add fettuccine and cook until just al dente according to package directions. Before draining, reserve at least 1/2 cup of the starchy pasta water. Drain and set aside.

Season the chicken: Slice the chicken breasts horizontally into cutlets for quicker, even cooking. Season both sides with salt and black pepper.

Sear the chicken: Heat 2 tablespoons olive oil and 1 tablespoon butter in a large skillet over medium-high heat. Add chicken and cook 3–4 minutes per side, until golden and cooked through. Transfer to a plate and tent with foil. Let rest, then slice into thin strips or bite-size pieces.

Start the sauce base: Reduce heat to medium. In the same skillet, add 3 tablespoons butter. Stir in the minced garlic and cook 30–45 seconds until fragrant. Do not let it brown.

Add the cream: Pour in the heavy cream and stir, scraping up any browned bits. Bring to a gentle simmer (small bubbles around the edges), then reduce heat to low. Simmer 2–3 minutes to slightly thicken.

Incorporate the cheese: Gradually whisk in the Parmesan in small handfuls, letting each addition melt before adding more. Keep the heat low to avoid splitting. If the sauce feels too thick, add a splash of reserved pasta water.

Season the sauce: Add salt, black pepper, and red pepper flakes if using. Taste and adjust. A tiny pinch of lemon zest can brighten the flavor without making it lemony.

Combine pasta and chicken: Add the drained pasta to the skillet and toss to coat, adding just enough reserved pasta water to create a silky, glossy sauce that clings to the noodles. Fold in the sliced chicken.

Finish and serve: Sprinkle with chopped parsley and a touch more Parmesan. Serve immediately while it’s creamy and hot.