Preheat the oven to 200°C, fan 180°C, gas mark 6. Place the potatoes in a large ovenproof dish. Add the onion, half the lemon slices, garlic and oil.
Season with plenty of salt and ground black pepper, then mix well until everything is coated in the oil. Bake in the oven for 20 minutes.
Stuff the fish with the remaining lemon slices and a few sprigs of the lemon thyme, then season well with salt and pepper.
Remove the potatoes from the oven and stir well, tuck in a few more sprigs of the thyme, then lay the fish on top of the potatoes and scatter over any remaining thyme leaves.
Return the dish to the oven and bake for a further 20 minutes, or until the potatoes are tender and golden, and the fish flakes easily with a fork. Serve hot.
*All information included in this recipe has been provided by Greenvale, British Gems Potatoes.