For the salt dough, place the flour and salt into a food processor. With the motor running, slowly add the egg whites, then the water, until the mixture comes together as a dough (you may not need all of the water). Tip the dough out onto a lightly floured work surface and knead for five minutes. Shape the dough into a sausage, cover with cling film and chill in the fridge for 30 minutes.
Meanwhile, for the lamb, preheat the oven to 150C/300F/Gas 2. Season the lamb with salt and freshly ground black pepper.
Heat a large frying pan over a medium to high heat and fry the lamb for 4-5 minutes, turning frequently, until browned all over. Remove the lamb from the pan and set aside to cool. Chill in the fridge.
Blanch the sea lettuce in boiling water for one minute. Drain.
Beat the butter in a bowl until softened, then beat in the sea lettuce. Set aside.
Roll half of the salt dough out to a 1cm/½in thickness. Scatter over half of the sea aster and sea purslane and dot teaspoonfuls of some of the butter and sea lettuce mixture into the middle. Top with the chilled lamb shoulder, placing it skin-side down. Brush the edge of the salt dough pastry with water, then fold the dough over the lamb and seal the edges with a fork.
Brush the top of the pastry with some of the beaten egg yolk and sprinkle with sea salt flakes and freshly ground black pepper.
Bake the pastry-covered lamb in the oven for 3 hours, then remove from the oven and set aside to rest.
When the pastry-covered lamb has cooked, increase the oven temperature to 180C/350F/Gas 4.
Roll the remaining salt dough out to a 1cm/½in thickness and cut into eight pieces.
Scatter over the remaining sea aster and sea purslane and dot teaspoonfuls of the remaining butter and sea lettuce mixture into the middle of each piece. Top each piece of dough with one of the potatoes. Brush the edge of each piece of pastry with water, then stretch the pastry around the potatoes, sealing the edges with a fork as before.
Brush the top of the pastry with the remaining egg yolk and sprinkle with sea salt flakes and freshly ground black pepper.
Bake the pastry-wrapped potatoes in the oven for 45 minutes.
Meanwhile, for the lamb jus, heat the butter in a frying pan until foaming. Add the lamb trimmings and fry for 2-3 minutes. Increase the heat, add two tablespoons of water and continue to cook for 2-3 minutes, or until the water has evaporated. Add another two tablespoons of water and continue to cook for 2-3 minutes, or until the water has evaporated. Add the shallots, garlic, thyme and bay leaf and stir until well combined. Fry for 4-5 minutes.
Pour in the wine, bring to a simmer and continue to cook until the volume of the liquid has reduced by half. Add the chicken stock and bring the mixture to the boil. Reduce the heat and simmer gently until ready to serve.
To serve, heat the butter in a frying pan and fry the sea beets and spinach for 2-3 minutes, or until wilted.
Carefully crack open the pastry crusts of the lamb and potatoes and discard. Carve the lamb into thin slices.
Strain the jus through a sieve.
Spoon the wilted sea beets and spinach onto serving plates and top with a couple of slices of the salt marsh lamb. Place the potatoes alongside. Drizzle over the jus and garnish with wild sorrel.