Fillet, descale and pin bone the sea bass (or ask your fishmonger to do it for you). Cut into 4 portions, and score lines in the skin to allow for even cooking.
Wash the Jersey Royals and cook them in salted boiling water until a knife goes through with no resistance. Then drain and crush them with a fork. Mix in the butter and parsley, and season.
To make the sauce vierge, heat the rapeseed oil in a pan. Gently sweat the shallot until soft, then leave it to cool to room temperature. Chop the olives and tomatoes and add to the oil along with the herbs, capers and a good squeeze of lemon. Season to taste.
Blanch the samphire in boiling water for 30 seconds.
Heat a glug of oil in a frying pan over a medium-high heat. Place the sea bass in, skin side down, and cook for 2 minutes, applying pressure to the flesh to stop it from curling up. Then flip over and cook for 2 more minutes. Add the drained samphire, a knob of butter and a squeeze of lemon to the pan.
To serve, add the potato to the plates, top with the sauce vierge, and then place the fish on top.