Sift the flour onto a work surface and make a well in the middle. Add the butter and salt to the well and work them together with your 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 125ml/4½fl oz ice-cold water and mix until well combined.
Roll the pastry into a ball, wrap in cling film and chill in the fridge for 20 minutes.
Flour the work surface and roll out the pastry into a 40cm x 20cm/15in x 7½in rectangle.
Fold one third of the dough to the centre, then fold the other third on top. Push the edges of the pastry with the rolling pin to seal.
Rotate the pastry a quarter turn. Roll the pastry out again 40cm x 20cm/15in x 7½in rectangle and fold in the same way as above. Chill in the fridge for 30 minutes.
Repeat the rolling and folding process two more times to give four rolls and folds in total. Wrap the pastry in cling film and chill in the fridge for at least 30 minutes.
For the filling, preheat the oven to 225C/450F/Gas 8.
Heat a frying pan until hot, add a little of the butter and heat until foaming.
Add the scallops and fry for 5-10 seconds on each side then remove from the pan and set aside on a plate. Add the monkfish slices to the pan and cook for 5-10 seconds on each side. Remove from the pan and set aside on a plate. Season, to taste, with salt and freshly ground black pepper.
Add the remaining butter, shallot and leek to the pan used to cook the scallops and monkfish and fry for 2-3 minutes. Deglaze the pan with the white wine and cook until the volume of the liquid has reduced by half.
Add the fish stock and cook for a further minute.
Add the cream and bring the mixture to the boil, then add the peas and cook until warmed through.
Stir in the chervil and season, to taste, with salt and freshly ground black pepper. Spoon a little of the mixture into each scallop shell, top with the scallops and monkfish then spoon over a little more sauce. Set aside to cool.
Meanwhile, roll the pastry out on a lightly floured surface to a 0.5cm/¼in thickness, then cut out four pieces of pastry big enough to cover the shells with a 2cm/1in border.
Brush the edge of each scallop shell with a little of the beaten egg. Lay the pastry gently over the top of the filled shell and press down around the edges to create a seal.
Brush with the remaining egg wash then mark the top lightly with a knife to decorate.
Place the scallop shells onto a baking tray in the oven and bake for 15-20 minutes, or until golden-brown and the pastry is cooked through.
Meanwhile, for the spinach, heat a frying pan until hot, add the remaining butter, spinach and water and fry for a couple minutes, or until wilted. Season with salt, freshly ground black pepper and nutmeg.
To serve, spoon some spinach into the centre of serving plates and place a scallop shell pie on top.