Rehydrate the fermented black beans by adding hot water to a bowl and set aside.
Mix all the seasoning ingredients together and set aside.
Combine all marinade ingredients with the beef and let it sit for 15–20 minutes.
Slice the onion, red, and green peppers into 1½ inch chunks.
Heat a pan with some oil, add the garlic and drained black beans. Stir fry for about 1 minute until fragrant, then remove from the pan. Avoid over-frying the beans to prevent bitterness.
Add more oil to the pan, stir fry the marinated beef for 4–5 minutes until browned, then set aside.
Stir fry the onion and peppers in oil for 3–4 minutes until soft but still slightly crunchy.
Add the garlic and black bean mixture, then the seasoning mix, and stir fry for 1 minute.
Return the beef to the pan and mix well.
Slowly pour in the cornstarch mixture to thicken the sauce. Stir to combine, then serve immediately.