Prep the beef and pasta. Pat the beef slices very dry with paper towels and season lightly with salt and pepper. Drying the meat removes surface moisture so the beef can sear instead of steam. Bring a large pot of salted water to a boil for the pasta.

Cook the pasta until just shy of al dente (about 2 minutes less than package directions). Reserve 1 cup of the starchy pasta cooking water, then drain. Finishing pasta in the sauce with a little reserved water lets the starch bind the sauce to the noodles for a glossy, clingy finish.

Heat a large heavy skillet (preferably stainless steel or cast iron) over high heat until hot. Add 1 tablespoon olive oil. Working in a single layer (do not overcrowd), sear half the beef slices for 45–60 seconds per side until browned but still medium-rare to medium; remove to a plate and tent with foil. Repeat with remaining beef using remaining 1 tablespoon oil. High-heat searing develops the Maillard crust that provides deep, savory flavor—short cooking keeps the strips tender.

Lower the heat to medium and add 2 tablespoons butter to the skillet. When the butter foams, add the thinly sliced onion and a pinch of salt. Cook, stirring occasionally, 8–10 minutes until onions are soft and beginning to caramelize. Gently caramelizing the onions creates sweetness and depth that balances the rich cream.

Add the minced garlic and smoked paprika; cook 30–45 seconds until fragrant. Toasting the paprika and garlic in fat blooms their aromas—don’t let them burn or they will turn bitter.

Sprinkle the flour over the onions and stir to coat for 1 minute. This creates a light roux which thickens the sauce and gives body—cooking the flour removes the raw taste.

Slowly pour in the beef stock while whisking (or stirring vigorously) to dissolve any lumps; then add the heavy cream, Dijon mustard, and Worcestershire sauce. Increase heat to bring the mixture to a gentle simmer. Simmer 4–6 minutes until the sauce thickens slightly—simmering concentrates flavor and reduces thinness without curdling the cream.

Taste and season the sauce with salt and pepper. If the sauce is too thick, whisk in up to 1/2 cup reserved pasta water a little at a time to reach desired consistency; this makes the sauce silkier and helps it cling to the noodles.

Return the seared beef and any collected juices to the skillet and warm through for 1–2 minutes—just long enough to finish cooking the beef to your preferred doneness. Overcooking will make the thin strips tough.

Add the drained pasta to the skillet and toss gently to coat in the sauce. If needed, add a splash more reserved pasta water to loosen. Finish by stirring in the optional 1 tablespoon butter for gloss and richness, then sprinkle with chopped parsley. Tossing everything together off direct high heat lets the pasta absorb the flavors and keeps the sauce smooth.

Plate immediately in warm bowls and serve. A hot plate keeps the sauce supple; garnish with extra parsley and a grind of black pepper.