Preheat oven to 425ºF (220ºC). Squeeze moisture from spinach, discarding liquid. Finely chop spinach. In large bowl, beat 1 egg with cream cheese until smooth. Mix in spinach, Parmesan, dill, mustard, lemon zest, lemon juice, garlic, half of both salt and pepper.
Cut salmon fillet in half, crosswise. Pat dry with paper towel and season with remaining salt and pepper. Place 1 sheet puff pastry on parchment-lined baking sheet. Beat remaining egg with 1 tbsp (15 mL) water and 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. Press together both sheets of pastry at edges to seal; roll and crimp border. Brush pastry with remaining egg wash. Cut vent holes on top.
Bake 35 to 40 min. until pastry is golden brown and crisp. Let stand 10 min. before slicing and serving with lemon slices.
Alternatively, use 1 pkg (397 g) unrolled puff pastry and roll pastry into two 10-in. (25 cm) squares on lightly floured work surface.