Prepare ingredients: slice onion thinly, clean and slice shiitake, julienne carrot, slice scallions on a bias. If beef is not pre-sliced very thin, place in freezer 20–30 minutes to firm then slice against the grain as thin as possible.

Heat a large skillet or wide sauté pan over medium-high heat. Add 1 tablespoon neutral oil and swirl to coat.

Add the sliced onion and carrots to the hot pan. Sauté 2–3 minutes until onion starts to soften but not fully translucent. Add shiitake mushrooms and continue to cook 2 minutes.

Push vegetables to the perimeter of the pan (or transfer to a bowl) and increase heat to high. Add half of the thinly sliced beef in a single layer and sear quickly, 30–45 seconds, stirring once — you want most pieces to brown but remain tender. Add remaining beef and repeat. Avoid overcrowding; quick high-heat searing preserves tenderness.

Return vegetables to the pan (if removed). Pour the prepared sauce over the beef and vegetables and toss to combine. Bring to a gentle simmer for 30–60 seconds.

Stir in the cornstarch slurry and simmer 1 more minute until sauce thickens slightly and coats the beef and vegetables. Taste and adjust seasoning with a little additional soy or a pinch of sugar if needed.

Finishing & Serving

Stir in sliced scallions and bean sprouts (if using) and cook 30 seconds until scallions are just wilted but still bright.

Transfer beef gyu to serving bowls over steamed rice or greens. Sprinkle toasted sesame seeds and add pickled ginger or fresh chili if desired. Finish with a light dusting of shichimi togarashi or black pepper to taste.

Serve immediately. Leftovers: cool, refrigerate in airtight container up to 3 days; reheat gently on skillet with a splash of water to loosen sauce.