Defrost the puff pastry according to the package directions.
For individual Wellingtons:
Preheat the oven to 425.
Cut the dough into three pieces (for the original recipe). Adjust accordingly if the recipe is multiplied. Cut the salmon into 3 fillets, about 5-7 ounces each.
Roll each piece of dough with a rolling pin or the side of a wine bottle, until each is big enough to wrap around the fillets. Use a sprinkle of flour to help it avoid sticking.
Fill the middle of the pastry rectangles with red onion slices and a few pinches of dill. Season the salmon with salt and pepper. Remove the skin if desired (we leave ours on since it is full of nutritious omega-3’s & adds moisture). Fold the dough up around the salmon, pinching & pressing it together to seal. Flip over and place on a baking sheet lined with parchment paper. Rub melted butter or olive oil onto the puff pastry and sprinkle with kosher salt.
Place in the oven for 20-25 minutes until they puff up and are golden.
For a full fillet
Preheat the oven to 425.
Lay the dough on a large board and roll it out into a piece that will wrap around the entire fillet. Aim for about 3-4 inches wider than the salmon to fold up and over.
Lay the slices of red onion and pinches of dill in the center of the dough. Season the salmon with salt and pepper and lay it flesh side down in the middle. Pull the puff pastry up and around, pinching slightly to secure in place.
Line a baking sheet with parchment paper. Flip the baking sheet over and place it on top of the folded up salmon. Lift up the board the salmon is on, press the board and baking sheet together gently and flip the entire ‘sandwich’ over. The wrapped salmon should now be sealed-side down on the baking sheet. Peel off the board carefully to not tear the pastry dough.
Drizzle on the melted butter and olive oil. Sprinkle with salt.
Bake for 20-30 minutes until the pastry is puffed and golden.
To serve: lift it up with the parchment and place on the serving dish. Slide out the paper. Cut into 1-2 inch slices to serve.