Preheat the oven to 180C/350F/Gas 4. Line a large baking tray with baking parchment.
Slice the cucumbers lengthways, scrape out the seeds with a teaspoon, and chop into 1cm/½in thick pieces.
Place the cucumbers in a mixing bowl with the mustard, honey, vinegar and finely chopped dill. Season well with salt and freshly ground black pepper and set aside.
Roll the pastry out to 40cm/16in x 30cm/12in on a lightly floured surface. Lay on the lined baking tray.
Lay one fillet in the centre of the pastry, pile half of the filling on top and then cover with the second fillet.
Brush the edges of the pastry with the mixture of milk and beaten egg.
Fold the pastry over the fish, sealing the edges by pressing them together, leaving a gap in the centre, so that you can see inside.
Brush the pastry with more beaten egg and milk and bake for 45 minutes.
Serve with the remaining cucumber mixture.