Prepare the lemongrass. Trim the ends, remove the tough outer layers, and finely mince the tender white and pale green parts.
Slice the beef. Cut the beef into thin slices, about 3 mm thick, against the grain.
Marinate. Combine the lemongrass, garlic, ginger, fish sauce, oyster sauce, sugar, oil, cornstarch, black pepper, and chilies. Add the beef and toss to coat. Cover and refrigerate for at least 30 minutes or up to overnight.
Fry the lemongrass. Heat 1 tablespoon of cooking oil in a wok or skillet over medium-high heat. Add 1 tablespoon of thinly sliced lemongrass and fry for about 2 minutes until fragrant and lightly golden. Remove and set aside.
Cook the beef. Increase the heat to high. Add half the remaining oil and half the beef in a single layer. Sear for 45–60 seconds, then toss for another 30–60 seconds until cooked. Transfer to a plate and repeat with the remaining beef and oil.
Cook the onion. Add the sliced onion to the pan and stir-fry for about 1 minute until slightly softened but still crisp.
Combine. Return the beef and its juices to the pan. Add 1 teaspoon fish sauce and the lime juice. Toss for 20–30 seconds.
Serve. Garnish with the fried lemongrass, scallions, and roasted peanuts. Serve immediately with steamed rice or rice vermicelli.
