Slice the beef thinly against the grain into strips. Add to a bowl with the soy sauce, cornflour, bicarbonate of soda, and pepper. Mix well and set aside to marinate briefly.
In a separate bowl, whisk together the Beijing sauce ingredients. Set aside.
Place a large wok over high heat. Add spray oil and the seasoned beef strips, and fry for 1 minute on each side, or until golden. Work in batches to prevent the meat from steaming. Remove and set aside.
Place the same wok back over medium-high heat, add a touch more spray oil, and add the onion and bell pepper. Stir-fry for 3 minutes, or until slightly soft and translucent. Add the garlic and cook for a further 45 seconds.
Return the beef to the wok along with the Beijing beef sauce. Mix well and cook for 1–2 minutes, or until the sauce has thickened and coats the beef and vegetables. Remove from the heat.
Rinse the jasmine rice, then add to a saucepan with the chicken stock and a pinch of salt (optional). Bring to a boil, reduce the heat to low, cover, and cook for 12 minutes. Turn off the heat, leave covered for 4 minutes, then fluff with a fork.
Divide the rice and Beijing beef between 5 x 750ml containers, either side by side or with the beef spooned directly over the rice. Garnish with spring onion and sesame seeds.