Boil potatoes in salted water for 15 mins until tender. Meanwhile, tip the spinach into a large colander in the sink.
Drain the potatoes over the spinach so the cooking water wilts it, then spoon the potatoes back into the pan. Mash, stirring in the lemon zest and juice and some seasoning, then fold through the trout flakes, squeezed-out spinach, capers and dill.
Tip half the breadcrumbs onto a large plate, then tip the potato mixture onto another plate and shape into a large disc. Flip the potato cake onto the crumbs, pressing the remaining crumbs onto the top.
Heat the grill and pour the oil into a large frying pan. Carefully slide the potato cake into the pan, then cook for 5 mins until the bottom is golden. Now grill for 4-5 mins until the crumbs on top are golden and the cake is hot through.
Serve hot with lemon wedges, tartare sauce and a green salad.
