Thinly slice the celery. Thinly slice the green onions (scallions). Finely chop the chives.
Whisk together the cider vinegar, lemon juice, ketchup, tomato paste, and salt. Then gradually whisk in the olive oil about 1 tablespoon at a time, until emulsified and creamy.
In a large serving dish, mix together the beans, celery, scallions, chives, dressing, feta crumbles, and fresh ground black pepper. If desired, top with additional feta crumbles and chopped chives as a garnish. Serve immediately or refrigerate until serving. It tastes even better 1 to 2 days later, so it’s great for making in advance!