Cook the potatoes in a saucepan of lightly salted boiling water for 12 minutes until tender, then drain and thickly slice. Toss with 4 tablespoons of the oil and the lemon rind and juice, season with salt and pepper and leave to cool a little.
Meanwhile, heat the remaining oil in a large frying pan. Add the fish fillets skin-side down, season and cook for 5 minutes. Turn over and cook for a further 3–5 minutes or until cooked through.
Stir the spring onions, chilli and herbs through the potatoes and serve with the fish.