Preheat the majority of the oil and fry the finely crushed leeks. Add the rice and cook with it for 2 minutes. Season with salt and pepper and top up to 1 cup hot water.
Sprinkle with lovage and let it boil until the rice swells. Transfer everything in a baking dish and top with the cleaned and salted mackerel. Add 2 more cups hot water in the pan, pour the oil in and put to bake in moderate heating oven to 356°F (180 °C).
Bake for about 40 minutes or until ready.