Place the chopped onion in a small bowl and cover with cold water. Let it sit for 5 minutes, then drain and rinse well to mellow its sharp flavor.
In a large mixing bowl, whisk together the red wine vinegar, olive oil, Dijon mustard, salt, and black pepper. Taste and adjust seasoning as needed. If the dressing tastes too tangy, stir in honey or maple syrup.
Add the beans, onion, cucumber, parsley, capers, and dried oregano to the bowl.
Toss everything thoroughly until evenly coated with the dressing.
Cover and refrigerate the Easy Bean Salad for at least one hour before serving to allow the flavors to develop.
Stir before serving and adjust seasoning if needed.