Trim the roots from the leeks, cut in half lengthways and run under cold water to clean.
Heat 1 tbsp vegetable oil in a large ovenproof dish over a medium heat. Fry the onion, leeks and sprouts until they start to colour. Add in the garlic and stir through. Remove from the heat and add in the thyme sprigs.
In a separate frying pan, heat 2 tbsp of vegetable oil over a medium heat. Once the oil is up to temperature, place the cod skin-side down and cook for 4-5 minutes or until the skin has gone crisp. Turn the fish over and cook for 3 minutes. Remove the fish from the frying pan. Place the fish on top of the vegetables in the ovenproof dish.
Add the butter and lemon to the dish, and season with salt and pepper to taste. Place in the centre of the preheated oven and bake for 10 minutes.
Carefully divide the cod and vegetables between 4 serving dishes and spoon over some of the remaining cooking juice.