Bring a large pot of salted water to a boil and cook the fettuccine according to package directions until al dente (about 9–11 minutes). Reserve 1 cup pasta water, then drain and set aside.

Pat the steak dry, season generously with salt and pepper, and let sit at room temperature for 10–15 minutes while the water heats.

Heat 1 tbsp olive oil in a heavy skillet or cast iron over medium-high heat. Sear the steak 3–5 minutes per side for medium-rare.

Remove steak to a cutting board and let rest 5–7 minutes. Slice thinly against the grain.

In the same skillet, add remaining 1 tbsp olive oil. Add shrimp seasoned lightly with salt and pepper. Cook 2–3 minutes per side until pink and opaque. Remove shrimp and set aside.

Lower heat to medium. Add butter and minced garlic to the skillet; sauté until fragrant, about 30–60 seconds.

Pour in the white wine (or broth) and scrape up any browned bits from the pan; simmer 1–2 minutes until reduced.

Pour in the heavy cream and bring to a gentle simmer; let simmer 3–5 minutes until the sauce starts to thicken. Stir occasionally.

Whisk in grated Parmesan cheese a handful at a time until the sauce is smooth and creamy. Season with salt, pepper, and crushed red pepper flakes.

If the sauce is too thick, add reserved pasta water 1/4 cup at a time to reach desired consistency.

Toss the cooked fettuccine into the sauce until coated, about 1–2 minutes. Add lemon zest if using.

Gently fold in the sliced steak and cooked shrimp just to warm through, about 1 minute.

Serve immediately with a sprinkle of chopped parsley and lemon wedges on the side.