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

Toss beef with half the sauce and let it sit for 10 minutes.

Sear beef in hot oil for 2 to 3 minutes until browned but still tender, then remove.

Stir-fry onion and bell pepper for 2 minutes.

Return beef to the pan, add remaining sauce, and toss briefly.

Serve immediately over rice.