Combine the mayo, lemon juice, capers and black pepper. Stir and leave to one side.
Whisk the egg in mixing bowl with a fork, then add the flaked fish and combine.
Add the spring onion, mashed potato, peas, mustard, parsley and salt & pepper. Mix well to combine.
Scatter the breadcrumbs onto a plate. Shape the fish cake mixture into roughly 8cm wide by 3cm high cakes, then press into the crumbs to evenly coat.
Heat a frying pan on a medium-high heat, add a generous drizzle of oil to coat the base.
Cook the fish cakes until golden and crispy, about 4-5 minutes each side.
Serve with the lemon & caper mayo. You can also add a side salad or poached eggs. Enjoy!