Adjust the oven temperature to 400°F (200°C).
Use salt, pepper, and dried dill to season the salmon fillet. Put aside.
Saute the minced garlic and diced onion in a pan until they soften. Toss in the frozen spinach and cook until thawed. Add salt and take off the heat.
Mix the cooled spinach mixture, fresh dill, cream cheese, lemon zest, and lemon juice in a mixing bowl. Thoroughly combine all ingredients by mixing them.
On a surface lightly sprinkled with flour, roll out the puff pastry sheet until it’s about 1/4 inch thick. Form the puff pastry into a rectangle large enough to wrap the salmon fillet completely.
Make sure there is approximately 1 inch (2.5 cm) of dough on each side of the doughnut, then equally distribute the spinach and cream cheese mixture over the center. Place the seasoned salmon fillet on top of the spinach and cheese mixture.
Before delicately folding the crust over the salmon, lightly brush it with the beaten egg yolk. To ensure a tight closure, use a fork to press down on the edges. On top of the puff pastry, spread the beaten egg yolk.
After lining a baking sheet with parchment paper, place the salmon wellington on top.
Bake for 20 to 30 minutes until the dough has turned a golden brown color and the salmon has cooked through.
After removing it from the oven, let it cool for a bit before slicing it and serving.