Beet Marinated Salad

2 (16 oz.) cans whole beets
½ cup white sugar
1 teaspoon prepared mustard
½ cup white wine vinegar
1 medium red onion, sliced thin
Drain beets, reserving ¼ cup liquid. Slice beets. Add onions to beets and toss. In a saucepan over medium heat, cook the sugar, mustard and reserved ¼ 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 to coat. Refrigerate until chilled.