Rub the haddock loins with 1 tablespoon of the oil, the lemon rind and fennel seeds. Place in a roasting tin, sprinkle with a little salt and pepper and leave to marinate in the refrigerator.
Put the tomatoes, cut side up, in one close-fitting layer in a roasting tin. Sprinkle with the garlic, dried oregano and salt and pepper. Drizzle with the balsamic vinegar and remaining oil and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 45 minutes.
Meanwhile, cook the potatoes in a large saucepan of salted boiling water for 18–20 minutes or until tender. Drain, crush lightly with the back of a fork and tip into an ovenproof dish. Spray with a little olive oil and season.
Remove the tomatoes from the oven and keep warm. Increase the oven temperature to 220°C (425°F), Gas Mark 7. Place the potatoes and the haddock loins in the oven for 10–15 minutes or until the fish flakes easily when pressed in the centre with a knife and the potatoes are slightly crispy. Serve the fish with the potatoes and tomatoes, garnished with oregano leaves.