Mix sifted flour, butter, and salt on a work surface until grainy.
Gradually incorporate 125ml ice-cold water until the dough is combined.
Wrap the pastry in cling film and chill for 20 minutes.
Roll pastry into a 40cm x 20cm rectangle on a floured surface.
Fold in thirds and seal the edges with a rolling pin.
Rotate the dough 90 degrees and repeat the rolling and folding process.
Chill for 30 minutes, then repeat rolling and folding twice more.
Wrap and chill the pastry for at least another 30 minutes.
Preheat the oven to 225°C.
Fry scallops for 10 seconds per side in butter and set aside.
Fry monkfish for 10 seconds per side, season, and set aside.
Sauté shallots and leeks in the same pan for 3 minutes.
Deglaze with wine and reduce by half.
Add fish stock and cook for one minute.
Stir in cream and boil, then add peas to warm through.
Add chervil and season the sauce before spooning into shells with fish.
Cover shells with 0.5cm thick pastry, seal with egg yolk, and brush tops.
Bake for 20 minutes until the pastry is golden-brown.
Wilt spinach in a pan with butter, water, and nutmeg.
Serve the pies on a bed of wilted spinach.