Place flour on a work surface and make a well in the center.
Work butter and salt into the well with fingertips, gradually drawing in flour.
Add iced water to the grainy dough and mix until incorporated without overworking.
Wrap dough in cling film and refrigerate for 20 minutes.
Roll pastry into a 40x20cm rectangle on a floured surface.
Fold top third down and bottom third up, then give a quarter-turn.
Repeat the rolling and folding process, then refrigerate for 30 minutes.
Perform two more rolling and folding turns, then refrigerate for at least 30 minutes.
Preheat the oven to 200°C.
Clean scallops and four shells, then slice scallops horizontally.
Fry shallots in butter until softened but not browned.
Add vermouth and fish stock, reducing the liquid by half after each addition.
Add cream and reduce the sauce by one-third.
Stir in tomatoes and tarragon, then season with salt and pepper.
Roll pastry to 3mm thickness and cut circles 1cm larger than the shells.
Fill shells with sauce and scallops.
Seal the shells with pastry lids using beaten egg.
Score pastry lids, brush with egg wash, and season.
Secure shells on salt piles on a tray and bake for 12 to 15 minutes.
Serve individual pies on fresh piles of salt.