Place the chopped onion in a small bowl and cover it with cold water. Let it sit for 5 minutes, then drain and rinse well to soften the sharp flavor.
In a large mixing bowl, whisk together the red wine vinegar, olive oil, Dijon mustard, sea salt, and black pepper until smooth. Add honey or maple syrup if you prefer a slightly sweeter dressing.
Drain and rinse the beans thoroughly under cold water until the liquid runs clear. Let them drain well.
Chop the cucumber, parsley, and capers while the beans drain.
Add the beans, onion, cucumber, parsley, capers, and dried oregano to the bowl with the dressing.
Toss everything well until all ingredients are evenly coated in the dressing.
Cover the bowl and refrigerate for at least 1 hour to allow the flavors to develop.
Before serving, stir the salad again and taste. Adjust with additional salt, pepper, or vinegar if needed. Serve chilled.