Add chopped onions to a small bowl and cover with cold water. Set aside for 5 minutes, drain, and then rinse. This step helps to remove some of the 'raw' flavor of the onion so that it does not overpower the salad.
Meanwhile, in the bottom of a large bowl, whisk together the vinegar, olive oil, mustard, salt and pepper. Taste the dressing and then adjust with more salt/pepper. If the dressing tastes too abrasive, whisk in 1 to 2 teaspoons of honey or maple syrup to balance out the vinegar.
Add the beans, drained onion, cucumber, parsley, capers and dried oregano. Toss well, cover and chill in the refrigerator at least 1 hour to allow the beans to marinate in the dressing.
The salad will keep stored in an airtight container in the fridge up to 4 days.