Mix the vinegar, sugar and water in a bowl. Add the seaweed and onion and mix well. Let it marinade for 1-2 hours. Serve the pickled seaweed on a plate and garnish with grated horseradish.
Parboiling the seaweed will remove any impurities.
Approx. ½ lb. edible seaweed (any variety), chopped ½ cup 12% distilled vinegar ¾ cup sugar 1¼ cups of water 2 tbs. fresh horseradish, grated 1 small red onion, thinly sliced ¼ tsp. salt