First, make the spinach filling. Melt the butter, and saute the diced onion until it's soft and translucent.
Add the spinach, and cook it until it just wilts. Add the garlic, and cook for 30 seconds. Season with black pepper, and add the cream cheese. After the cream cheese melts, add the parmesan cheese and the breadcrumbs. Mix everything together and remove from heat.
Season the salmon fillets with salmon and pepper. Then roll out the pastry sheet on your counter, cut it into halves (or in a size that's good enough to fit the salmon and the filling).
Place each salmon fillet in the center of the puff pastry sheet and leave about 2 inches around the edges. If the puff pastry sheet is not big enough, you can roll it out.
Top each salmon fillet with the spinach mixture using a spoon, spread it evenly. Brush the edges of the puff pastry sheets with a beaten egg, and fold it as shown in the image below.
Place the salmon on a greased or lined with parchment paper baking sheet seam side down. And make crosshatch slits on the top of each Wellington using a sharp knife.
Bake at 200°C (390°F) for 30 minutes or until the pastry is golden.