Rinse and drain the white beans. Drain the tuna very well.
Whisk olive oil, lemon juice, Dijon, garlic, salt, black pepper, and red pepper flakes in a small bowl.
Add beans, tuna, celery, red onion, parsley, and capers to a large mixing bowl.
Pour in the dressing and gently fold to combine without smashing the beans.
Taste and adjust seasoning with more lemon, salt, or pepper as needed.
Serve right away or chill 10 minutes. Spoon over greens, toast, or crackers.