1. Sprinkle a large pot of water with salt and add onion, celery, carrot, bay leaf, and peppercorns. Bring to a boil and add potatoes. Cook until you can stick a fork into the potatoes easily. Be careful not to overcook them or the potatoes will start to fall apart. Once they are cooked, drain the water from the pot. Slice the potatoes into thin disks. Place them and the vegetables in a bowl. Quickly add olive oil and vinegar while still warm. Now set aside.
2. Next, drain the tuna from its can. In a separate bowl, mix the tuna, capers, red onion, and olives. Now add this to the bowl of potatoes and toss. Add additional salt, pepper, olive oil, or vinegar if you need it. To serve the salad, place a handful of arugula in three bowls (or tupperware, makes a great lunch) and spoon the salad on top.