Marinate beef into soya sauce, brown sugar, and sesame oil for at least 15 minutes.
Heat oil in a pan, add minced garlic, stir fry it for few seconds, make it sure it doesn't burn, add beef stir fry it for at least a minute until it turns golden brown. Add brocoli and hoi sin sauce, continue stirring it for few minutes until brocoli is done. Add black pepper to taste stir fry it for few seconds. Serve hot in a bed of noodles or steamed rice.