Remove as many bones as you can from the fish and cut it into 1½ in / 4 cm. cubes. Mix together the sugar, spices and seasoning, and use to coat the salmon. Roll out the pastry, and use half of it to line a pie dish, about 9 in / 23 cm. in diameter. Scatter the couscous over the base. Peel, core and slice the apple paper thin, dip it into lemon juice mixed with water to stop it browning, and then blot on kitchen paper. Use half the slices to line the base of the pie dish. Place the salmon pieces on top, and scatter the dried fruit in the spaces between, if using. Grate the lemon zest over the salmon, and top with the remaining apple slices. Dot with the butter. Roll out the remaining pastry and use to cover the pie making sure the edges are well sealed. Use any pastry trimmings to decorate the pie. Cut a small hole in the top, and keep it open with a roll of foil or greaseproof paper (or use a pastry funnel). Brush with beaten egg and milk, place the pie on a baking sheet, and bake in a preheated oven at 180°C / 350°F / Mark 4 for 20 minutes.
Meanwhile, infuse the saffron in a little boiling water, mix it with the cream, and beat in the eggs. Remove the pie from the oven, and turn the heat up to 200°C / 400°F / Mark 6. Remove the roll of foil or paper and carefully pour the saffron custard mixture into the pie, stopping when it shows signs of overflowing. Return the pie to the oven for 10–15 minutes to set the custard.