Prepare the sprouts: Trim the ends of the Brussels sprouts and shred them finely using a knife or food processor.
Heat the oil: In a large skillet or wok, heat olive oil over medium heat.
Sauté aromatics: Add sliced onion and cook for 2–3 minutes until softened. Stir in minced garlic and sauté for 30 seconds until fragrant.
Cook the sprouts: Add shredded Brussels sprouts to the pan. Stir well and cook for 5–7 minutes, stirring frequently, until tender but slightly crisp.
Season: Pour in soy sauce, sprinkle chili flakes, and mix thoroughly. Cook for another 1–2 minutes.
Finish: Drizzle fresh lemon juice over the stir-fry and give it a final toss.
Serve: Serve hot as a healthy side dish or pair with rice or noodles for a complete meal.