Toss the beans, celery, onion, parsley and capers gently in a large bowl.
In a separate bowl, whisk together the lemon juice, mustard and mayonnaise.
Gradually whisk in the olive oil. Add salt and pepper.
Pour the dressing over the bean mixture and toss gently until evenly coated.
At this point you can refrigerate the salad for several hours or overnight. Flavors will marry well after sitting.
An hour or so before serving, remove from the refrigerator and toss gently.
Drain the tuna and place in a small bowl. With a fork, break up the tuna into small chunks.
Add to the bean salad and mix gently, so as not to break tuna more.
Serve on salad greens.
Leftover salad will still taste good the next day.
Recipe by Jane O'Riordan