Toss beef with soy sauce, fish sauce, oyster sauce, lime juice, sugar, garlic, and black pepper.

Marinate for 30 minutes.

Heat oil in a wok or skillet over high heat until shimmering.

Sear beef in a single layer, shaking the pan to brown all sides, 3-4 minutes.

Add red onion and cook for 1 minute.

Serve over watercress with lime wedges.