Preheat an oven to 400°F. Pat the salmon dry with paper towels, then season generously with 2 tsp kosher salt and ¼ tsp black pepper. Grate the zest from 1 lemon on top. Allow the salmon to sit out at room temperature while you prepare the Swiss chard.
Remove the ribs from the Swiss chard, then cut into ¼-inch pieces. Set aside. Next, roughly chop the leaves.
Melt 2 Tbsp unsalted butter in a large skillet over medium heat. Add the Swiss chard stems and chopped shallot, then cook for 3-5 minutes. Add 2 chopped garlic cloves, then cook for 1 minute more. Add chopped Swiss chard leaves. and 1 tsp kosher salt. Cook, stirring occasionally, until the leaves have wilted and cooked down, about 3-4 minutes. Stir in 5oz Boursin until melted and creamy.
Roll the thawed pastry out on a lightly floured surface so that it is twice as long as the salmon fillet.
Place the salmon seasoned side down in the middle of the pastry. Spoon the creamy filling on top, then spread into an even layer over the salmon. Try not to let it fall over the sides. Carefully fold the puff pastry over the salmon, beginning with the longer sides first, then the shorter sides, encasing it completely. You can use a little egg wash to help ‘glue’ the pastry shut or simply press the pieces together firmly.
Place the puff pastry seam side down on a parchment-lined baking sheet. Brush with a beaten egg, then use a sharp knife to cut diagonal slits along the top of the pastry. Sprinkle with flaky sea salt and a little black pepper. Transfer to the preheated oven, then cook for 25-30 minutes, until the puff pastry is golden brown.
In a small bowl, combine ½ cup (120g) crème fraîche, 1 Tbsp fresh lemon juice, 2 tsp chopped dill, and 1 tsp Dijon mustard. Stir well. If you want the sauce slightly thinner, you can add a splash of water.
Allow the salmon wellington to cool slightly, then cut into slices about 1½-inch thick and serve with the crème fraîche sauce. Enjoy!