Combine flour, salt, and butter in food processor until mixture resembles coarse meal. Add water and egg. Process again until dough begins to form a ball. Remove from processor and shape into two discs by hand. Wrap dough in plastic wrap and refrigerate for 30 minutes.
Melt butter in a large skillet over medium-high heat and cook spinach until wilted.
Add garlic and continue cooking for 30 seconds. Season with salt and pepper. Drain spinach in a sieve, pressing to remove water, and set aside.
Place oven rack in the lower position and preheat oven to 220°C (425°F).
On a floured work surface, roll out the two discs of dough to about 5 mm (1/4 inch) thick in the shape of the fillet. The dough will be much bigger than the salmon.
Place one sheet of dough onto a parchment paper–lined baking sheet. Centre the fish on the dough. Cover with dill and lemon zest and season with salt and pepper. Spread spinach over fillet and scatter almonds on top. Arrange hardboiled eggs in a line down the centre of fillet.
Brush edges of dough with beaten egg. Cover with second sheet of dough, pressing edges together to seal. Trim excess dough. Use a pastry brush to coat dough with beaten egg. Make three slits in top crust between eggs to allow steam to escape during baking.
Bake salmon in oven until pastry is golden, about 40 minutes. Let salmon cool for 10 minutes before serving.