In a bowl, marinate beef for 15 minutes with fish sauce, lime juice, and black pepper.
Heat oil in a wok or frying pan over high heat. Once the oil is hot, add garlic and lemongrass and stir-fry until fragrant (about 1-2 minutes).
Add in the onion and the beef (with the marinate). Let the beef sear for a minute.
Then add in turmeric powder, sugar, soy sauce, and fish sauce, and quickly toss all ingredients together.
Add in red bell pepper, and a bit of water if needed. Stir-fry for another 2-3 minutes, being careful not to overcook the beef.
Remove from heat. Squeeze in some more lime juice, add a pinch of black pepper, and garnish with cilantro.
Serve the Vietnamese stir-fried beef with steamed rice.