Mix beans and onion together in a serving bowl; set aside. Combine remaining ingredients in a saucepan and heat until sugar dissolves; pour over bean mixture. Refrigerate at least 12 hours prior to serving.
This tastes just like the 3 bean salad my mom used to make. A kids we always loved it, and it was always requested at family gatherings and other functions. So simple yet yummy. NOTE: We cut the sugar to 1/4 cup (4Tablespoons) and added a pinch more salt. For the vinegar we used red wine vinegar; but any flavor would work; sometimes some ingredients can be used interchangeably.