Bring a large pot of salted water to boil. Add potatoes and cook until halfway done (about 10-15 minutes). Place fish fillets in a colander and place on top of the boiling potatoes. Cover colander with foil or lid and let the fish steam while the potatoes continue to cook (about 10 minutes).
Remove fish from colander and shred into pieces on a cutting board.
Drain potatoes from water, let cool, and mash.
Combine mashed potatoes and shredded fish. Add in parsley, Worcestershire sauce, mustard, eggs, lemon zest, salt, pepper, and flour. Mix well.
Dust a large cutting board or baking sheet with flour. Using your hands, scoop ¼ cup of mixture and flatten into small disk about ¾ inch tall. Place on cutting board. Repeat with remaining mixture (you should make about 10 cakes).
Place cutting board with fish cakes in fridge to let them firm up for an hour.
When ready to cook, heat olive oil in a large frying pan. Add fish cakes and cook 3-4 minutes per side, until nicely crisped.
To make the sauce, combine sour cream and horseradish. Dollop on fish cakes.