Clean the mackerel well and sprinkle with salt and pepper. Optionally, you can remove the heads, tails and fillet them. Combine 3 1/3 tbsp (50 ml) of wine, 3 1/3 tbsp (50 ml) oil and zest of half a lemon and pour this marinade over the fish so that it is evenly seasoned.
Leave the mackerel to stand and cool for about half an hour. Heat 4 tablespoons of oil separately and sauté the crushed leeks.
The potatoes are peeled and cut straight in the tray in which you will bake them. Add the leeks on top and sprinkle with all the seasonings to taste. Between the potatoes and leeks, arrange the mackerel and pour in the remaining wine, oil and some warm some water.
Put the potatoes and mackerel in the oven to bake at 356°F (180 °C) under foil for about 45 minutes. Then remove the foil and bake until done.
Serve the dish with a little diced fresh parsley.