For the vinaigrette:
1/3 cup apple cider vinegar
2 tablespoons white sugar
1/4 cup olive oil
1/2 teaspoons salt
1/4 teaspoon pepper
Salad ingredients:
1 15-ounce can kidney beans, rinsed and drained well
1 15-ounce can cannellini beans, rinsed and drained well
2 cups fresh green beans, tough ends removed and chopped into 1" pieces
1/2 red, white or sweet onion, finely chopped
Make the vinaigrette: Add dressing ingredients to a lidded glass jar. Put the lid on tightly and shake vigorously to combine. Set aside.
Bring a pot of salted water to a boil. Add the green beans and cook for 2 minutes or until crisp-tender. Place the green beans in a bowl of ice water to cool, then drain and dry the beans.
Add the green beans, cannellini beans, green beans and onions to a nonreactive mixing bowl. Pour vinaigrette over the beans and toss to combine. Let the salad rest for at least an hour before serving or store, covered and chilled, for up to 3 days.