Rinse & Ready (3 min) : Drain beans in a fine-mesh strainer, rinse under cold water, then pat very dry with towels so the vinaigrette clings well.
Cure the Crunch (5 min) : In the chilled bowl, toss the sliced onion with 1 tbsp vinegar and a pinch of salt. Let stand while you prep the veggies to soften sharpness and add brightness.
Blend the Bright (2 min) : Whisk olive oil, remaining vinegar, lemon juice, thyme, sumac, black pepper, and lemon zest. Stir in optional 1 tsp caper brine.
Fold & Fluff (4 min) : Add beans to the bowl with onions; pour in ¾ of the dressing. Fold gently until beans look lightly glossy.
Load the Color (3 min) : Add cucumber, tomatoes, bell pepper, capers, and half the olives. Fold in the remaining dressing until evenly coated.
Creamy Finish (1–2 min) : Sprinkle in feta, parsley, and mint. Toss once or twice so the feta stays craggy and visible.
Rest & Refresh (10 min, optional) : Let sit 10 minutes (room temp or lightly chilled) to meld flavors. Taste and adjust seasoning before serving. Top with remaining olives and a few feta crumbles.