1.Boil, steam or microwave potato until tender; drain. Using potato masher, crush potato roughly in large bowl with butter and oil. Cover to keep warm.
2.Meanwhile, make salsa verde. Blend or process parsley, mint, olive oil, capers, mustard, lemon juice, anchovies and garlic until just combined. Transfer to medium jug; season to taste, whisk before pouring over fish.
3.Cook fish, in batches, on heated oiled grill plate (or grill or barbecue) until browned both sides, but still fairly rare in the middle; overcooking will make it dry.
4.Divide rocket and potato between serving plates; top with fish, drizzle with salsa verde.
If the potatoes that you purchase for this recipe are young and quite small, scrubbing them well (rather than peeling) will be sufficient. Tuna is at its best if browned both sides