1: Preheat the oven to fan 200°C/gas 6. Cook the potatoes on a baking tray in the oven for 50 minutes to 1 hour, or until soft right through. Remove and set aside to cool. (Turn off the oven.)
2: Spoon out the potato ﬂesh into a large bowl and mash it roughly. Add the spring onions, gherkins, capers, mustard, lemon zest and parsley. Season generously with salt and pepper and mix well. Drain the tuna, then gently fold it in, being careful not to break it up too much.
3: Divide the mixture into 4 portions and shape into patties. Chill in the fridge for at least 2 hours to ﬁrm up (overnight is ﬁne).
4: When you’re ready to cook the ﬁsh cakes, heat the oven to fan 200°C/gas 6. Line a baking tray with baking parchment and spray it with oil. Have the beaten egg and breadcrumbs ready in separate shallow bowls; season both with salt and pepper.
5: Coat each ﬁsh cake ﬁrst in the beaten egg and then in the breadcrumbs, making sure they are coated all over. Place the ﬁsh cakes on the prepared tray and spray each one with oil. Cook for 15 minutes, then ﬂip each ﬁsh cake over and cook for another 15 minutes.
6: Serve hot, with lemon wedges and watercress, dressed with herb dressing if you wish.