Heat a large nonstick skillet over medium-high heat. Add onion and cook until softened, about 2 minutes. Add in spinach, garlic powder, half of the salt and pepper and cook, stirring constantly, about 3 minutes. Stir in cream cheese until melted; remove from heat. Stir in feta cheese, dill, lemon juice and zest. Set aside.
Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment.
Roll 1 piece of puff pastry onto a lightly floured surface to a size just slightly larger than the salmon fillet. Spread half of the spinach mixture in the shape of the salmon fillet over the pastry. Place salmon on top of spinach mixture and sprinkle with remaining salt and pepper. Add remaining spinach mixture on top of salmon and place second pastry sheet on top. Trim excess pastry from edges; fold edges of pastry all around salmon to create a seal.
Transfer salmon to the prepared baking sheet; brush evenly with egg.
Bake in the preheated oven until golden brown and fish is cooked through, about 30 minutes. An instant read thermometer inserted into the center of salmon should read 145 degrees F (63 degrees C.) Let stand 5 minutes before serving.
