Put the potatoes into a saucepan and cover with cold water. Bring to the boil, turn down and simmer for 10 - 15 minutes, then drain.
While the potatoes are cooking, mix the tartare sauce, olive oil, water & juice of one lemon in a large bowl.
Add the warm potatoes and toss to coat them.
Toss through the herbs and set aside.
Pat the barramundi fillets dry with a clean tea towel and, using a sharp knife, lightly score the skin, enough to go through the skin but not cut into the flesh.
Season the fish liberally on both sides.
Add the rapeseed oil to a pan over medium-high heat, then add the fillets, skin-side down. Press the fillets down gently to keep them from curling up.
After 3 minutes, or when the skin is crisp and golden (just lift one up and have a peek), flip them over. Add the butter, as it melts and browns, baste the fish constantly and cook the other side for a further 2 minutes or until just cooked.
Squeeze over lemon juice and swirl to combine.
Place a generous amount of the salad on each plate, put the fish on top and drizzle with the pan juices.