In a skillet over medium heat, melt the butter. Add the leek and cook for 3 minutes, until just tender. Add the wine and cook for another 2 to 3 minutes, until the wine is nearly evaporated. Add the spinach and cook, stirring, until just wilted, about 1 minute. Stir in 1/4 cup of cream cheese and cook until it has completely melted, about 2-3 minutes. Remove from heat and cover to keep warm.
In a small jar with a lid, combine the balsamic vinegar, brown sugar, garlic, salt, pepper and olive oil. Cover and shake until mixed well. Brush dressing onto both sides of salmon fillets.
Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper. On a floured surface, cut the puff pastry sheet in half and roll out into two rectangles (about 7 x 8 inches). Spoon half of the spinach mixture onto center of each pastry rectangle and top with salmon fillets (skinned-side up). Fold the long sides of the puff pastry over the fish fillet, then the ends to completely enclose the fish (feel free to cut away any extra pastry that doesn't fit). Gently press edges together to seal.
Place, seam sides down, on the baking sheet and brush the top of the puff pastry with an egg. Using the back of a knife, score a criss-cross pattern on top.
Bake for 30 to 35 minutes or until crust is golden brown.
Melt the remaining cream cheese in the microwave, then stir in dill, lemon zest and juice. Serve with salmon. Enjoy!