Whisk soy sauce, fish sauce, oyster sauce, lime juice, sugar, garlic, and black pepper.

Toss beef with the marinade for 10 minutes.

Heat oil in a large skillet or wok over high heat.

Sear beef in a single layer, shaking the pan as it browns, until cooked to preference.

Add red onion for the final minute so it softens but keeps bite.

Serve immediately with rice and crisp vegetables.