Boil the pasta. Bring a large pot of salted water to a boil. Cook pasta until just shy of al dente. Reserve 1 to 1.5 cups of starchy pasta water, then drain.

Prep your aromatics. Thinly slice the onion. Mince the garlic and grate the ginger. Slice scallions for garnish.

Slice the beef. If not pre-sliced, freeze steak for 20 minutes to firm up, then slice thinly across the grain. Pat dry and season lightly with salt and pepper.

Sear the beef. Heat a large skillet over medium-high. Add oil. Sear beef in two batches for 1–2 minutes per side until just browned. Transfer to a plate. Do not overcook.

Soften the onions. Lower heat to medium. Add butter to the same pan. Sauté onions until soft and lightly golden, 4–6 minutes. Add garlic and ginger; cook 30 seconds until fragrant.

Build the Korean BBQ base. Stir in gochujang, soy sauce, and brown sugar. Let it bubble for 30–60 seconds to thicken slightly and coat the onions.

Make it creamy. Pour in heavy cream and 1/2 cup pasta water. Stir until the sauce is smooth and slightly thickened. Taste and adjust with more soy for salt, sugar for sweetness, or gochujang for heat.

Combine pasta and beef. Add cooked pasta and the seared beef (plus juices) to the skillet. Toss to coat, adding more pasta water as needed to create a glossy, silky sauce that clings to the noodles.

Brighten and finish. Stir in rice vinegar and sesame oil. Toss for 30 seconds. Season with pepper and a pinch of salt if needed.

Garnish and serve. Top with scallions and sesame seeds. For extra heat, sprinkle gochugaru. Serve hot.