1. Bring water to a boil in a large, heavy saucepan. Add farro and simmer, partially covered, over medium-low heat 30 minutes. Turn off heat, cover pot and let farro rest 5 minutes. Fluff with a fork and transfer to a large serving bowl to cool, about 10 minutes.
2. Add tuna, raisins, olives and capers; stir well to break up tuna. Stir in lemon juice, mustard and oil. Add arugula and gently toss to combine; season with salt and pepper to taste.