Place the roast slices on a large platter or in a baking dish.
Peel the onion, dice finely. Clean and wash the spring onions, cut into thin rings and cover and set aside. Dice the cucumber.
Mix the vinegar with the mustard, salt, pepper and a pinch of sugar. Fold in the onion and cucumber. Withhold oil. Spread the mixture on the roast. Cover the marinated roast slices in a cool place for 1 hour.
To serve, arrange roast slices on plates, drizzle with a little marinade and sprinkle with spring onions and fresh herbs.