Preheat the oven to 220C/425F/Gas 7. Prick the potatoes with a knife then place in the oven and bake for 45 minutes until cooked through.
Meanwhile, remove the scallops from the shell, clean well and trim, leaving the main flesh of the scallop.
Place the trimmings into a saucepan with the fish stock and warm through.
Clean four of the deep halves of the shells then place in the oven for a couple of minutes to dry out. Remove the shells from the oven and place them onto a baking tray on mounds of salt to keep them level.
Remove the cooked potatoes from the oven and leave to cool for a few minutes before cutting them in half and scooping out the flesh.
Pass the potato flesh through a ricer into a bowl then add the egg yolks and 25g/1oz of the butter and work with a wooden spatula until smooth.
Season lightly with salt and freshly ground black pepper then place into a piping bag and pipe carefully around the edge of the four shells set on the baking tray.
Heat a saucepan until just hot, then melt 50g/2oz of the butter, then stir in the flour and cook for two minutes to make a roux.
Take the pan off the heat. Pour the warm fish stock through a sieve into the roux, stirring all the time. Add the double cream, then put the pan back on the heat and bring to a simmer. Cook for a couple of minutes, whisking frequently until thickened. Season with salt and freshly ground black pepper and keep warm.
Pre-heat the grill on a high setting.
Season the scallops with salt and freshly ground black pepper. Heat a frying pan until hot, add the remaining butter and cook the scallops on each side for 1-1½ minutes until golden-brown. Remove the scallops from the pan and place on kitchen paper to drain.
Bring a pan of salted water to the boil, add the vegetables and blanch for two minutes, or until just tender. Remove the vegetables from the pan.
Place the vegetables into each scallop shell, so that they are visible over the edge of the shell. Place two scallops in each shell on top of the vegetables. Add some sprigs of the fennel top.
Cover generously with the roux and fish stock sauce, and sprinkle with gruyere and breadcrumbs.
Place the baking tray with the scallops on, under the grill for 1-2 minutes, until the sauce is lightly glazed and the potato border pale golden.
To serve, pile salt into the centre of four plates. Carefully place the scallop shell on top of the salt and serve immediately.