Bake the potatoes in a hot oven until tender – about 1 hour – then set aside to cool. Meanwhile, prepare the jam. Sweat the onion in the butter till soft and then add the fruit, sugar and vinegar and cook until reduced to a jammy consistency. Mush it with a fork or back of a spoon. Next, to make the croquettes, sweat the leek till soft in some oil over a very low heat. While that’s sweating, remove the flesh from the warm potatoes and place in a bowl. Add in the soft leek and the flaked mackerel, and mix well but do not blitz in a food processor or the spuds will go too starchy. Just mash and mix. Season well, and roll into balls about 3cm round and chill in the fridge. About a half hour before cooking, remove them from fridge and roll them all in flour, egg and then breadcrumb. Chill again. You can get this bit done several hours in advance. Fry in batches in oil over a medium heat until golden on the outside. If using them for a starter, serve with some pretty leaves and a spoonful of the jam on the side. For canapés, just serve them on a plate with some mini forks or toothpicks and a bowl of the ‘jam’ dip in the middle.