- Preheat the oven to 190 °C (375 °F).
- Line an upside-down baking sheet with parchment paper.
- In a saucepan, cook the mushrooms and shallots in half the butter until dry. Add the lime zest and juice.
- Continue cooking until the liquid has evaporated. Season with salt and pepper. Set aside in a bowl.
- In the same pan, add the remaining butter and wilt down the spinach. Add the wine and cook until almost dry. Set aside.
- Roll half the pastry to form a rectangle a little larger than the salmon fillet. Place the dough on the baking sheet.
- Pat dry the salmon fillet and place it on the dough. Season with salt and pepper. Spread the mushroom mixture over the salmon. Spread the spinach on the mushrooms.
- Roll out the remaining dough. Brush the dough around the fish with the egg and cover with the second dough rectangle. Trim the excess to give the dough the shape of a fish. With a fork, press the edges of the fish. Make decorations (scales, eye, tail) with the dough scraps. Brush with the beaten egg.
- Bake for about 55 minutes and let stand for 5 minutes before serving.