Preheat your oven to 400°F.

Season the salmon fillet with salt, pepper, and dried dill. Set aside.

In a pan, sauté the chopped onion and minced garlic until they soften. Add the frozen spinach and cook until heated through. Season with 2 teaspoons of salt and remove from heat.

In a mixing bowl, combine the cooled spinach mixture with fresh dill, cream cheese, lemon zest, and lemon juice. Mix until well combined.

On a lightly floured surface, roll out the puff pastry to about 1/4 inch thickness. Cut the pastry into a rectangle large enough to enclose the salmon fillet completely.

Spread the spinach and cream cheese mixture onto the center of the pastry, leaving about an inch of pastry on all sides. Place the seasoned salmon fillet on top of the spinach mixture.

Carefully fold the pastry over the salmon, sealing the edges with the beaten egg yolk. Use a fork to press down on the edges to ensure a tight seal. Brush the top of the pastry with the remaining egg yolk.

Place the wrapped salmon on a baking sheet lined with parchment paper.

Bake for 25-30 minutes or until the pastry is golden brown and the salmon is cooked through.

Remove from the oven and let it rest for a few minutes before slicing and serving.