Preheat oven to 400 degrees F.
Add oil, garlic, and spinach to a skillet and heat over medium heat until wilted, about 5 minutes.
Add cream cheese, mustard, dill, and spinach to a mixing bowl and beat until creamy, about 90 seconds.
Gently flour parchment paper or silicone mat big enough for a large sheet pan, and unfold puff pastry. Gently roll out until slightly thinned and stretched and smooth.
Pat dry the salmon filet well. Do not skip this step. Salt and pepper top and bottom of salmon filet(s), then place onto one half of the puff pastry, leaving an inch on each side and enough to wrap over the top once the filling is added.
Spoon spinach filling over top of salmon.
Add egg to a small bowl and whisk with 1 teaspoon milk. Gently brush over edges of puff pastry.
Fold puff pastry over salmon and filling, then fold edges back over and seal together.
Crimp edges using finger or fork like you would a pie crust.
Using a knife, cut slits into top of puff pastry in any design of your choice, cutting just through the pastry.
Brush the top of the pastry with the remaining egg wash.
Move to sheet pan.
Bake in the preheated oven for 30 minutes or until salmon is cooked through.
Remove from heat and let rest for 10 minutes before serving.
Garnish with fresh dill and serve with lemon slices.