Preheat the oven to 200 ° C. Cut the fillet into 20 thin slices and carefully flatten it with the knife. Clean the onions and cut into short pieces.
Put the remaining ingredients in an ovenproof bowl and mix thoroughly. Pull the fillet slices through the sauce and wrap a fillet slice around each onion piece. (The moisture in the meat holds the rolls together). Place in the soy mixture and cover and refrigerate for at least 30 minutes.
Simmer in the oven (225 ° C, top / bottom heat) for 10 - 15 minutes, brushing the sauce several times. Serve hot or cold.