Boil the potatoes in a saucepan of salted water for 12-15 minutes, or until very tender, then drain and return to the pan. Return to the heat for a couple of minutes to drive off any excess moisture. Set aside until cool.
Pour enough water into a saucepan to reach 5cm/2in up the sides. Add the lemon, bay leaf and onion and bring to the boil. Add the fish and prawns and simmer for 3-4 minutes, or until just cooked through, then remove from the heat and leave the fish to cool in the cooking liquid.
Meanwhile, for the beurre blanc, heat the shallot and white wine in a saucepan until bubbling. Reduce the heat and simmer until there is only about 50ml/2fl oz liquid left in the pan.
Whisk in the butter, a little at a time, waiting until most of the butter has melted and incorporated into the mixture before adding more.
Once all of the butter has been added, add a squeeze of lemon juice and strain the mixture into a clean saucepan. Stir in the chives and season, to taste, with salt and freshly ground black pepper. Cover with cling film and set aside at room temperature until needed.
When the fish and potatoes are cool, flake the fish into a large bowl, discarding any skin and bones. Roughly chop the prawns and add them to the bowl. Add the chopped dill and spring onions and season with a squeeze of lemon juice.
Add the potatoes and season, to taste, with salt and freshly ground black pepper. Carefully combine the mixture with your hands, taking care not to break up the fish too much.
Form the mixture into 6 equal-sized patties. Chill in the fridge on a plate for 5 minutes.
Meanwhile, heat half of the butter in a frying pan over a medium heat until foaming. Add the chard and sauté for 3-4 minutes until wilted, then season, to taste, with salt and freshly ground black pepper. Drain onto kitchen paper then clean the pan with a piece of kitchen paper and return it to the heat.
Add the remaining butter to the pan. When foaming, add the fish cakes, in batches if necessary, and fry for 2-3 minutes on each side, or until golden-brown on both sides and piping hot throughout.
Just before serving, remove the cling film from the beurre blanc and heat through.
To serve, spoon the chard onto serving plates, top each portion with a fish cake and spoon the chive beurre blanc around the edge.