Step 1Bring a saucepan of water to a boil. Meanwhile, prepare an ice bath. Add the haricots verts to the hot water and boil just until crisp but tender, about 30 seconds. Strain and shock in the ice bath, then drain and set aside.
Step 2In a small saucepan, boil the potatoes until just tender, about 15 minutes. When cool enough to handle, drain and cut each in half.
Step 3Place the prepared vegetables in a bowl. Add the tuna, capers, celery leaves, celery, parsley and lemon juice. Gently toss and season with salt to taste.