Instructions:

1. Preheat the oven to 400°F and line a baking tray with baking paper.
2. Bring a medium saucepan of salted water to the boil. Add both potatoes and cook for 20–25 minutes until tender. Remove from the heat, drain and steam-dry for 5 minutes. Roughly mash using a potato masher.
3. Transfer the mashed potato to a medium mixing bowl, add the salmon or mackerel, spring onions, half the parsley and the beaten egg. Mix to combine and season well before stirring in the lemon zest and a squeeze of juice. Using your hands, mold the mixture into 6 fishcakes and transfer to the lined baking tray. Bake for 30–35 minutes until golden, turning halfway through cooking.
4. While the fishcakes are in the oven, grate the celeriac and add to a bowl. Mix in the milk kefir yogurt, remaining parsley and a squeeze of lemon juice. Season to taste.
5. Serve the fishcakes alongside the celeriac slaw.