Put the new potatoes in a large saucepan with 1–2 tablespoons of the oil. Place over a medium-low heat and cover with a tight-fitting lid. Cook for about 20 minutes, shaking the pan frequently to move the potatoes around. When done, the potatoes should be cooked and crispy golden. Remove from the pan and sprinkle with sea salt.
Meanwhile, mix together the mayonnaise with the chervil and garlic.
Heat the remaining oil in a large frying pan over a medium-high heat. Season the fish with salt and pepper and cook, flesh-side down, for 1 minute before turning carefully and frying for a further 2–3 minutes until the skin is crispy. Squeeze over the lemon juice and serve immediately with the crispy potatoes and garlicky mayonnaise.