Place rhubarb in a shallow, heat-proof bowl. In a small saucepan, combine sugar, balsamic vinegar, salt, and mustard seeds; bring to a boil. Cook until sugar dissolves. Pour mixture over rhubarb and let sit at room temperature for at least 3 hours before using. The pickled flavoring improves if refrigerated overnight. Leftover pickling liquid can be refrigerated for future use.
Pickled Rhubarb 2
5 rhubarb stalks, sliced thinly
1 cup sugar
1 cup apple cider vinegar
1 tsp. salt
Bring sugar, vinegar, and salt to a boil. As soon as all the sugar is dissolved, pour over the sliced rhubarb. Let stand for at least 3 hours. Then put them in refrigerator until well chilled. These will keep covered for a couple of weeks in the refrigerator.