With a nod to the past where dinner party fare was fancy and delicate, Salmon Wellington is a spectacularly beautiful dish yet incredibly easy. A perfect excuse to dust off the fine china, polish the silver, set out the crystal goblets, and light the candles.
Preheat the oven to 425°F and line a baking sheet with parchment paper.
In a pan over medium heat, melt butter. Add the garlic and onions, cooking until translucent. Add mushrooms and saute until brown but not juicy, about 6 minutes. Add Madeira, spinach, salt, and pepper; continue cooking until spinach has wilted. Remove from heat.
Add dill and Parmesan to the spinach mixture and combine.
On a cutting board, smooth out a sheet of thawed puff pastry. Place the salmon in the middle of the pastry and season both sides with salt and pepper.
Place several spoonfuls of the spinach mushroom mixture on top of the salmon, smoothing it out so that it does not spill over the sides.
Fold the edges of the puff pastry over the salmon and spinach, starting with the long sides and then the shorter ends. Trim any excess pastry from the ends, then fold the ends on top. Flip the puff pastry-wrapped salmon over and transfer to the baking sheet.
With excess pastry, create decorative shapes and adhere to the top of the Salmon Wellington with a dab of water. Brush the beaten egg on the top and sides of the pastry.
Bake for 20-25 minutes, until the pastry is golden and the salmon is cooked through.