Sprinkle the fish liberally with fine salt and leave in the fridge to cure for 20 minutes. Wash under cold water, pat dry and cut in half.
Preheat the oven to 180C/160C Fan/Gas 5.
Pour the beef stock and red wine into a pan and bring to a simmer until thickened and reduced to about 150ml/5fl oz total. Set aside.
Rinse the cockles well under running cold water and discard any with cracked or damaged shells. Discard any open cockles that do not close when given a sharp tap. Heat a medium saucepan with a lid to a high temperature, add the cockles, garlic, shallot and thyme. Pour over the white wine, add a pinch of sea salt and cover with the lid. Cook for 1 minute, or until the cockles just open. Drain in a colander set over a bowl, reserving the liquid. Discard any cockles that fail to open.
Pick out three-quarters of the cockles from their shells, reserving the remainder in their shells. Discard the thyme, garlic and shallot.
Pour the reserved white wine liquid through a fine strainer into a clean saucepan and set over a medium heat to simmer and reduce in volume by about one-third. Add the cream and cook for another 5 minutes or until reduced and thickened. Whisk in the cold butter, add the chives and season with salt and pepper.
Heat a non-stick oven-proof frying pan over medium heat, add a splash of oil and cook the fish skin-side down for a few minutes. Transfer the pan to the oven for 4 minutes. It should feel firm when pressed lightly. Remove the pan from the oven using an oven glove and rest the fish skin-side down for 1 minute.
Put the kale into a small pan with a splash of water and a pinch of salt. Put a lid on the pan and cook for 2 minutes until just tender, then turn off the heat and drain.
Re-heat the beef sauce and drop all the cockles, picked and shelled, into it.
Pour the white wine sauce onto two serving plates, place the kale in the centre. Squeeze a little lemon juice over the fish and then arrange on top of the kale skin-side up. Garnish with the picked cockles and shell-on cockles with a little of the beef sauce. Serve immediately.