1. Toss the beef strips in the garlic and ginger and allow to marinate for 10 minutes.
2. In a small bowl mix the cornflour and water together to make a smooth paste. Stir in soy sauce, brown sugar and chilli. Set aside.
3. Heat 1 tsp of oil in large fry pan over high heat, add half of the beef strips and stir-fry until browned then remove from pan. Repeat with 1 tsp of oil and remaining beef strips.
4. Heat remaining tsp of oil, add choy sum stems and stir-fry 1-2 minutes then return beef strips along with sauce mix.
5. Cook, stirring, until sauce boils and thickens. Serve with rice.