Preheat oven to 220ºC (425ºF). Squeeze moisture from spinach, discarding liquid. Finely chop the spinach. In large bowl, beat 1 egg with the cream cheese until smooth. Mix in spinach, Parmesan, garlic, dill, mustard, lemon zest, lemon juice, salt and pepper.
Cut across the fillet to divide it into 2 portions of equal length. Pat dry with paper towel. Place 1 sheet puff of pastry on parchment-lined baking sheet. Separate remaining egg (reserve egg white for another use); beat yolk with 1 tbsp (15 mL) water; Brush 1-in (2.5-cm) border around edges of pastry. Place 1 piece of salmon in centre of pastry. Spread half the spinach mixture on the salmon. Top with second piece of salmon. Spread remaining spinach mixture on top of fish. Cover with second sheet of puff pastry. Pressing out any air pockets, seal top and bottom pastry sheets around edges. Trim excess pastry around bottom sheet. Roll and crimp sealed edges. Brush pastry with remaining egg wash.
Use the tip of a paring knife to score diagonal lines across surface of pastry (when the pastry puffs up during baking, score lines will reveal as lighter colour than eggwashed pastry.) Cut 3 vent holes on top.
Bake 35 to 40 min., until pastry is golden brown. The temperature of the salmon should register 70˚C (158˚F) on an instant-read thermometer. Let stand 10 min. before slicing and serving with lemon wedges.
Alternatively, use 1 pkg (397 g) unrolled puff pastry and roll into two 10-in. (25-cm) squares on lightly floured work surface.
Ask at the fish counter to have skin removed from salmon fillet to save on prep time at home.