Preheat the oven to 350°F (180°C).
Scale and wash the fish.
Generously butter an earthenware dish large enough to hold the fish and the vegetables. Season the fish all over generously with salt and pepper. Arrange small new potatoes and onions around it. If the vegetables are not new, first boil them briefly, drain, and season with table salt.
Drizzle the melted butter over the fish and vegetables and cook in the oven, spooning frequently with the cooking liquid—if you like you can add 1 tablespoon of olive oil to it. The onions should comprise one quarter of the total garnish. Make sure that they stand firmly on the base of the dish so that they are in constant contact with the buttery cooking juices. This way, they will turn a nice golden color.
Just before serving the fish in the cooking dish, sprinkle the fish and vegetables with the freshly chopped parsley. On the side, serve the lemon quarters to squeeze over the fish when it is served into warmed plates.
Poaching and grilling are two other cooking methods that are very suitable for cod. Cut it into 1-in. (2-cm) slices and serve it with one of the sauces recommended for poached or grilled fish.
Paul Bocuse shares 500 easy-to-prepare, traditional French recipes in his new collection The Complete Bocuse, published by Flammarion.