Bring a large pan of water to the boil. Add the potatoes and cook until tender. Drain the potatoes and mash them, but don’t worry about a few lumps. Leave until cool enough to handle. This is a great way to use up leftover mashed potato from the night before.
Preheat the oven to 180°C/gas mark 4. Line a baking tray with non-stick baking parchment. In a large bowl, mix together the mash, horseradish, smoked mackerel, parsley, lemon zest, a good grind of black pepper and the egg yolk. Once everything is mixed well and combined, divide the mix into 8. Shape into patties.
Beat the egg and egg white in a bowl, then pop the breadcrumbs in another. Season the breadcrumbs with pepper. Dip each pattie into the egg, then into the breadcrumbs. Place on your lined baking tray. Place in the hot oven for 15-20 minutes or until hot through, then serve with veggies or salad.