Oven-baked Mackerel Stuffed with Pesto Mash

First cook the potatoes in boiling salted water for 20 minutes.

Test them with a skewer and when they're absolutely tender, drain them well. Leave them in the saucepan and cover with a clean tea cloth to absorb some of the steam. Next, add all but 1 tablespoon of the pesto to the potatoes, then use an electric hand whisk to mash them – start with a slow speed to break them up, then go on to high until you have a smooth, lump-free purée.

Now fold in the spring onions and taste to check the seasoning. Next, make the topping for the fish by dropping cubes of bread into a food processor or liquidiser with the motor switched on, then follow with the porridge oats until all is uniformly crumbled.

To prepare the fish, wipe them inside and out with kitchen paper, lay them on the foil-lined baking tray and make three diagonal cuts about 1 inch (2.5 cm) in depth all along the top side of the mackerel. Spoon the pesto mash into the body cavities, pack it in neatly, then fork the edges to give some texture.

Now brush the surface of the fish with olive oil, scatter it with the crumbs and finally add 1 dessertspoon of olive oil to the remaining pesto and drizzle it over the crumbs using a teaspoon. Now it's ready for the oven: bake for 25 minutes on a high shelf, then serve with lemon quarters and sprigs of flat-leaf parsley.