Drain beets, reserving 1/4 cup liquid, and slice into 1/4 to 1/2 inch slivers. Add onions and toss.
In a saucepan over medium heat, cook the sugar, mustard and reserved 1/4 cup liquid until dissolved. Add vinegar and bring to boil; remove from heat and allow to cool.
Pour over the beet slices and onions, toss and refrigerate for 4 to 6 hours. Remove from refrigerator and serve at room temperature.
