Method

1. Start by making a thick cheese sauce. Add the butter, flour and milk to a heavy-based saucepan and whisk together over a medium heat, then continue whisking as the sauce heats through, making sure to scrape down the sides
2. Allow to boil for 2–3 minutes to cook off the flour, whisking all the time. Mix in the grated cheese until smooth, then set aside
3. In another pan, heat a little olive oil until it begins to smoke, then add the spinach and stir until wilted. Drain immediately to remove any excess liquid, then stir the cooked spinach into the cheese sauce, along with the pine nuts and garlic
4. Stir to combine, then allow to cool (this mixture will keep in the fridge until ready to assemble and bake)
5. Preheat the oven to 200°C/gas mark 6
6. To assemble the dish, unroll the first pastry sheet on to a large non-stick baking tray, and place the salmon fillet in the centre. Season well with salt and pepper
7. Spread a thick layer of the cheese and spinach sauce on top of the salmon, then top with the second sheet of pastry. Tuck the top sheet around the salmon, then brush the edges with a little egg yolk, squeezing the two sheets together to seal
8. Carefully trim off any excess pastry around the edge; these trimmings could be used to decorate the top of the pastry, if liked (cut out your desired shapes and stick on to the top pastry with a little of the beaten egg)
9. Brush the whole pastry with the egg, and bake in the preheated oven for 40 minutes, until golden all over. Serve immediately