Preheat the oven to 190°C. Heat the fish stock in a saucepan until steaming but not boiling.
Layer the sliced potatoes and onion in a large greased ovenproof dish, adding the thyme and half the chives plus a little seasoning between each layer. Mix the lemon zest and juice with the stock and pour over the vegetables. Cover the dish with aluminium foil, then bake for 45 minutes or until the potatoes are almost tender.
Meanwhile, cut the fish into large, bite-sized chunks, discarding any skin and bones. Uncover the ovenproof dish and arrange the fish on top of the potatoes in a single layer. Re-cover the dish and return it to the oven. Bake for a further 15 minutes.
Remove the foil and scatter the peas over the fish and potatoes. Finish with the tomatoes, cut-side up, then sprinkle with coarsely ground black pepper. Cover the dish and return it to the oven for a final 10 minutes.
Scatter the rest of the snipped chives over the fish and vegetables as a garnish, then serve.
• Replace the onion with a thinly sliced bulb of fennel. • Fresh or frozen broad beans can be used instead of peas.