Pat the sirloin steak dry with paper towels. Season generously on both sides with 1 tsp kosher salt and 1/2 tsp black pepper. Let sit at room temperature for 15-20 minutes while you prepare other ingredients.

Heat a large, heavy-bottomed skillet (preferably cast iron) over high heat until smoking hot. Add 1 tbsp olive oil.

Carefully place the seasoned steak in the hot skillet. Sear for 3-4 minutes per side for medium-rare, or longer to your desired doneness. For a 1-inch thick steak, this usually means an internal temperature of 130-135°F for medium-rare. Use tongs to sear the edges briefly if desired.

Remove the steak from the skillet and transfer to a cutting board. Tent loosely with foil and let rest for at least 10 minutes. This step is crucial for juicy, tender steak.

While the steak rests, bring a large pot of heavily salted water (with 1 tbsp kosher salt) to a rolling boil. Add the fettuccine and cook according to package directions until al dente (typically 8-10 minutes).

Before draining, reserve 1 cup of the pasta cooking water. Drain the pasta and set aside.

Return the same skillet used for the steak to medium heat (do not clean it, the browned bits add flavor). Add 2 tbsp unsalted butter.

Add the sliced cremini mushrooms and cook, stirring occasionally, until softened and browned, about 5-7 minutes.

Add the thinly sliced yellow onion and cook until translucent and slightly caramelized, about 5 minutes.

Stir in the minced garlic and cook for 1 minute until fragrant.

Pour in 1 cup beef broth and 1 tbsp Worcestershire sauce, scraping up any browned bits (fond) from the bottom of the skillet with a wooden spoon. Bring to a simmer and reduce slightly, about 2-3 minutes.

Reduce heat to low, then stir in 1/2 cup heavy cream. Bring to a gentle simmer, but do not boil vigorously.

Stir in 1/2 cup freshly grated Parmesan cheese until melted and smooth. Taste and adjust seasoning with salt and pepper as needed. If the sauce is too thick, add a splash of the reserved pasta water.

Slice the rested steak against the grain into thin strips.

Add the cooked fettuccine and sliced steak to the skillet with the sauce. Toss gently to coat everything evenly. If needed, add more reserved pasta water a tablespoon at a time to reach your desired consistency.

Stir in 1/4 cup chopped fresh parsley.

Serve immediately, garnished with extra Parmesan cheese and fresh parsley.