Preheat the oven to 230C/210C Fan/Gas 8.
Heat half of the butter in a large frying pan over a medium heat. When the butter is foaming, add the mushrooms and fry for 4-5 minutes, or until they have released most of their water and it has evaporated from the pan. Season, to taste, with salt and freshly ground black pepper, then transfer to a plate lined with kitchen paper.
Return the frying pan to the heat, add the last of the butter and then wilt the spinach leaves and season, to taste, with salt and freshly ground black pepper. Add to the plate with the mushrooms, set aside until cool, then chill in the fridge.
Roll out both pieces of pastry onto a lightly floured work surface to create two rectangles about 5cm/2in longer and wider than the salmon fillets.
Place one sheet of pastry onto a large flat baking tray and lay one salmon fillet on top, flesh side up. Season well with salt and freshly ground black pepper.
Top the salmon fillet with the mushrooms and then the spinach, then lay the second salmon fillet on top of that, flesh side down, pressing down gently.
Brush the edges of the pastry with some of the beaten egg yolk. Carefully roll the second sheet of pastry onto the coulibiac using a rolling pin to transfer it. Press the edges of each sheet of pastry together to seal and crimp at the edges.
Brush the surface of the pastry parcel all over with the remaining beaten egg yolk. Using a sharp pair of scissors, snip the top of the pastry at intervals to make marks resembling fish scales.
Bake the salmon coulibiac for 25 minutes, then reduce the oven temperature to 200C/180C Fan/Gas 6 and bake for another 12-15 minutes, or until the salmon fillets are just cooked through and the pastry is risen and golden-brown.
Remove the salmon coulibiac from the oven and transfer it to a serving platter. Set aside to rest for 10 minutes, then carve into slices at the table and serve with buttered new potatoes and a dressed green salad.