Add a little oil to a heavy-bottomed pot or pressure cooker and brown the beef in batches. Brown them on all sides. Remove and set aside.
Mix together all the sauce ingredients in a bowl. Taste and adjust seasoning. You want a good balance of heat, sourness, sweetness and saltiness.
Add the beef pieces to a pressure cooker or pot. Pour the sauce and stir through. Cover and cook over a low-medium heat for 20-25 minutes in a pressure cooker or 1-1 ½ hours if using a regular pot (see notes). Test a piece - cook until it’s falling apart and tender enough to pull apart with a fork.
Once it’s cool enough to handle, remove the meat pieces and shred them. I find this easiest to do with my fingers.
Add the shredded pieces back to the pot and cook over a high heat to reduce the sauce to the right consistency. You want it sticky enough to coat the pieces but not too runny that it makes the buns soggy.
To serve, halve the buns and toast the cut sides. Add a good amount of the beef, top with shredded cabbage and cucumber and serve immediately.