Place the fish fillets in a buttered dish, season to taste and cover with foil. Bake in oven until the fish is cooked through. Allow to cool slightly, removing any skin and bones then flake with a fork.
Boil potatoes until tender but do not over cook, drain well.
Transfer to a mixing bowl then add butter and grated Beechwood, mix together well. Add beaten egg and mix together thoroughly.
Add chopped spring onions and flaked fish, season to taste with salt and pepper.
Mix together breadcrumbs and parsley, spread out crumbs on a plate.
Divide potato mixture into 4 and roll firmly into a ball. Insert a cube of Mozzarella into each ball and roll again to ensure cheese is encased in potato.
Press balls gently to form cake shape then coat each cake in breadcrumbs and place on greaseproof paper.
Chill for 10 minutes to allow cakes to firm up
Heat oil in a large heavy sauté pan over medium heat. Cook fish cakes for 4-5 minutes on each side until hot and golden brown.