Have the butcher cut your roast in 1/8 inch slice against the grain. OR
do it at home. if you don't have good cutlery, have the butcher do it. it
won't be worth it! Cut off the slice of fat on the bottom. separate the
two distinct sinews. (you can tell, the grain goes one way on one, and
the other way on the other). Trim out the "gristle" that is between
them. Slice the two sinews against the grain into 1/8 inch slices.
Mix the soya, and remaining ingredients to make the marinade.
Marinade the meat slices for 6 or 8 hours in the fridge...