Slice the beef thin. For easy slicing, chill the beef in the freezer for 20–30 minutes. Slice against the grain into very thin strips. Thin slices cook quickly and stay tender.

Whisk the marinade. In a bowl, combine soy sauce, brown sugar or honey, gochujang, rice vinegar, mirin (if using), garlic, ginger, sesame seeds, and sesame oil. If you want a glossier, thicker sauce, whisk in 1 teaspoon cornstarch.

Marinate the beef. Toss the beef with half the marinade. Let it sit 10–20 minutes while you prep vegetables. If using grated pear or apple, mix it into the marinade for extra tenderness and subtle sweetness.

Cook the noodles. Boil according to package directions until just shy of done. Drain and rinse briefly (especially if using udon or rice noodles) to prevent sticking. Toss with a splash of oil and set aside.

Prep the veggies. Slice onion and bell pepper thin. Shred Napa cabbage or bok choy. Slice green onions. Keep them ready by the stove; this cooks fast.

Sear the beef in batches. Heat 1 tablespoon oil in a large skillet or wok over high heat until shimmering. Add half the beef in a single layer. Cook 1–2 minutes per side until browned with some caramelized edges. Remove to a plate and repeat with remaining beef, adding more oil if needed. Do not overcook.

Stir-fry the vegetables. In the same pan, add a bit more oil if dry. Add onion and bell pepper; stir-fry 2–3 minutes until crisp-tender. Add cabbage or bok choy; cook 1–2 minutes until just wilted.

Add noodles and sauce. Return beef (and any juices) to the pan. Add cooked noodles and the remaining half of the marinade. Toss over medium-high heat for 1–2 minutes until the noodles are coated and glossy. If the sauce looks tight, splash in 2–3 tablespoons water.

Finish and garnish. Taste and adjust: more soy for salt, a pinch of sugar for balance, or more gochujang for heat. Top with green onions and extra sesame seeds. Serve hot.