Cook the potatoes in a large saucepan of boiling, salted water until tender; usually 15-20 minutes. Poach the fish in the milk in a saucepan at the same time over a medium heat for 6-8 minutes until just cooked.
Pre-heat the oven to 200 degrees a this point and line a baking tray with nonstick baking paper as well.
Drain the fish and reserve to one side. Drain the potatoes and allow to cool and dry out a little before mashing with the fish, wholegrain mustard, sweetcorn and seasoning in a large mixing bowl.
Shape the mixture into 8 even-sized patties. Dust in the flour, shaking off any excess, then dip in the beaten egg. Coat in the breadcrumbs and arrange on the lined baking trays. Bake for 15-20 minutes until golden brown and crispy on the outside. Remove and serve on plates with the lettuce leaves as a garnish.