01 - Combine chicken with lemongrass, garlic, shallots, chili, fish sauce, soy sauce, brown sugar, 1 tbsp vegetable oil, and black pepper in a large bowl. Toss thoroughly to coat evenly. Cover and marinate for at least 20 minutes, or refrigerate up to 4 hours for deeper flavor penetration.
02 - Heat 2 tbsp vegetable oil in a large skillet or wok over medium-high heat. Add sliced onion and sauté until fragrant and translucent, approximately 2 minutes.
03 - Add marinated chicken to the skillet. Cook, stirring frequently, until chicken develops a golden brown exterior and is cooked through, about 8–10 minutes.
04 - Taste the dish and adjust seasoning with additional fish sauce or pepper if needed. Remove from heat.
05 - Transfer to a serving platter, garnish with spring onions. Serve hot alongside steamed jasmine rice, fresh cucumber slices, and lime wedges.