For the marinade, juice and zest the 2 oranges. Heat the vinegar with the sugar until it caramelises, add the orange juice and zests, then the Grand Marnier. Cool. Place the duck tenderloins in this mixture and chill for 3 hours.
For the salad, remove the skin and pith of the 2 oranges to obtain the segments. Peel and finely chop the red onions, slice the spring onions thinly. Combine a