Drain the tuna and rinse the white beans in a strainer. Pat the beans dry gently. Chop the cucumber and pepper into small bite-size pieces and finely dice the red onion.

Whisk the Greek yogurt, olive oil, lemon juice, Dijon mustard, and black pepper in a large bowl until smooth. Wait to add salt because the tuna, beans, and mustard may already supply enough.

Add the white beans and fold them through the dressing with a spatula. Use a light hand so the beans stay intact rather than turning into a paste.

Add the drained tuna, cucumber, pepper, red onion, and parsley. Fold just until the dressing coats everything, leaving a few larger tuna flakes for texture.

Taste and add salt only if needed. Divide into two equal portions. Serve right away, or refrigerate promptly in covered containers until needed.

For a packed lunch, keep the salad chilled with an ice pack. Carry crackers, bread, or greens separately so they do not soften in the dressing.