In a pan over medium heat, melt butter. Add onion, saute for a few minutes and drop in chopped garlic, continue cooking until onion till soft.
Add the spinach, salt and pepper, cooking for about 2 minutes. Add the breadcrumbs, cream cheese, parmesan cheese and dill, stirring until mixture is evenly combined. Remove from heat and set aside to cool.
On a table top, smooth out the puff pastry sheet (dust some flour beneath puff pastry). Place the salmon fillet in the middle of the pastry and season with salt and pepper.
Place half of spinach mixture on top of the salmon fillet, smoothing it out so that it does not spill over the sides.
Brush the edges of the puff pastry with egg wash and fold the edges of the puff pastry over the salmon and spinach. Place seam sides down on a baking sheet lined with parchment paper. Repeat for the remaining spinach mixture to the second piece of salmon fillet.
Using a sharp knife, make crosshatch slits on top of the pastry. Brush the egg wash on top of the pastry.
Bake in preheated oven (second lower rack) at 220 deg C for about 25-30 minutes, until pastry is golden brown.