For the rough puff pastry, place the flour in a mound on a clean work surface and make a well in the centre.
Place the butter and salt in the well and work them together with the fingertips of one hand, gradually drawing the flour into the centre with the other hand.
When the cubes of butter have become small pieces and the dough is grainy, gradually add the iced water and mix until it is all incorporated, but don’t overwork the dough – you should have a marbled effect with the butter without mixing it in properly.
Roll the mixture into a ball, wrap in cling film and refrigerate for 20 minutes.
Flour the work surface and roll out the pastry into a 40x20cm/16x8in rectangle.
Fold the top third down to the centre, then the bottom third up and over that. Give it a quarter-turn.
Roll the block of pastry into a 40x20cm/16x8in rectangle as before, and fold it into three again. These are the first two turns. Wrap the block in cling film and refrigerate it for 30 minutes.
Give the chilled pastry another two turns, rolling and folding as before. This makes a total of four turns. Wrap the pastry in cling film and refrigerate for at least 30 minutes before using.
For the tart, preheat the oven to 200C/400F/Gas 6.
Remove the scallops from their shells, retaining four of the deep shells (the other shells can be discarded). Rinse the removed scallops and the reserved shells and pat them dry with kitchen paper. Cut each scallop in half horizontally to form two thinner scallops. Set aside.
Heat a frying pan until medium hot, add the butter and shallots and fry for a couple of minutes, or until softened but not browned.
Add the vermouth and cook until the volume of liquid has reduced by half. Add the fish stock and cook until reduced by half again.
Add the cream and cook until reduced by one-third.
Add the tomatoes and tarragon, then taste and add salt and freshly ground black pepper to taste.
Roll the pastry out to about 3mm thick, then using one of the shells as a template, cut around the shell, leaving a 1cm/½in border around the edge.
Spoon some of the sauce into the four shells, top with the scallops and the remaining sauce.
Brush the edge of the shells with the beaten egg, then lay the pastry over the top and press to seal around the edge.
Mark the top of the pastry with a knife in lines, mimicking the lines on a scallop shell. Brush with the remaining beaten egg, and season with a little salt and black pepper.
Place four piles of salt onto a baking tray then place the scallop shells on the piles so that they sit flat. Bake for 12-15 minutes, or until the pastry is golden-brown and crisp.
Remove and serve the scallop pie on another little pile of salt on a plate.