Toss sliced beef with bicarbonate of soda. Leave 15 mins (no longer!), then rinse and pat dry.
Add soy sauce, cornflour, oil, and water. Mix well and leave 10–15 mins.
Stir together black bean sauce, water, and sugar.
Heat a wok until very hot. Add oil, then beef in one layer. Cook 30 seconds, remove.
Stir fry onion and peppers for 1–2 mins. Add garlic and cook 20 seconds.
Pour in the sauce mixture and let it simmer.
Add the cornflour slurry and stir until glossy.
Return beef, toss for 30 seconds. Finish with sesame oil if using.
