Boil potatoes in salted water for 15 minutes, drain, and dry over heat.
Simmer lemon, bay leaf, onion, fish, and prawns in water for 4 minutes.
Simmer shallot and white wine in a saucepan until reduced to 50ml.
Whisk in chilled butter cubes a little at a time until incorporated.
Strain sauce, stir in chives, season, and cover with film at room temperature.
Flake fish, chop prawns, and combine in a bowl with dill and spring onions.
Mix in potatoes and seasoning by hand without over-breaking the fish.
Form mixture into 6 patties and chill in the fridge for 5 minutes.
Sauté chard in 25g butter for 4 minutes until wilted and season.
Fry fish cakes in remaining 25g butter for 3 minutes per side until golden.
Reheat the beurre blanc gently just before serving.
Serve fish cakes on chard with the chive sauce spooned around.