Preheat your oven to 180°C.

Begin by preparing the leeks: trim the roots and cut them in half lengthwise. Rinse under cold water to remove any dirt trapped between the layers.

Heat 1 tablespoon of vegetable oil in a large ovenproof dish over medium heat. Add the sliced onions, leeks, and halved Brussels sprouts. Cook until they begin to soften and develop a golden hue. Stir in the chopped garlic, allowing it to infuse the vegetables with its aroma. Add the thyme sprigs, then remove the dish from the heat.

In a separate frying pan, heat 2 tablespoons of vegetable oil over medium heat. Once the oil is hot, place the cod portions skin-side down. Cook for 4–5 minutes, or until the skin is golden and crisp. Flip the fish gently and cook for another 3 minutes to lightly sear the other side. Remove the cod from the pan and arrange it over the vegetables in the ovenproof dish.

Dot the butter around the dish and drizzle the lemon juice evenly over the fish and vegetables. Season generously with salt and pepper. Transfer the dish to the preheated oven and bake in the center for 10 minutes, allowing the flavors to meld and the fish to cook through.

To serve, carefully divide the cod and vegetable mixture among four plates. Spoon some of the flavorful cooking juices over each serving for added richness. Pair with roasted new potatoes for a complete and satisfying meal.