Cook the new potatoes in salted boiling water for 6–8 minutes until almost cooked. Drain and place in an ovenproof dish with the rosemary, drizzle with 1 tablespoon of the oil and season with salt. Roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes, or until golden brown. Turn the oven off but leave the potatoes in it to keep warm while you cook the fish.
Heat another tablespoon of the oil in a large frying pan. Season the fish with salt and pepper and place it, skin-side down, in the hot pan. Cook the fish on the skin side for 3–4 minutes, or until the skin becomes crispy. Turn the fish over and cook for a further minute. Remove the fish from the pan and keep warm while you make the dressing.
Put the lemon rind and juice, cream and sage into the pan and stir well to combine. Add a little water if the sauce becomes too thick.
Pour the sauce over the fish and serve with the roasted new potatoes.