Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.
Place the garlic, shallots and mushrooms in a food processor and pulse to chop finely.
Heat the butter in a medium skillet over medium heat. Add the chopped mushroom mixture and cook, stirring occasionally, until softened, 3 to 5 minutes. Stir in the white wine; reduce over high heat until the mixture is dry. Off the heat, stir in creme fraiche, Dijon and salt and pepper to taste. Set aside to cool slightly.
Sprinkle the salmon fillets with salt and pepper on both sides. Set aside.
On a lightly floured work surface, roll 1 sheet of the puff pastry into a 12-by-8-inch rectangle, and then cut into quarters. Place a small stack of spinach leaves on the short side of two of the quarters. Top with a salmon fillet and about 2 tablespoons of the mushroom mixture. Brush the pastry edges lightly with egg wash, top with the remaining quarters and crimp the edges. Place the packages on the prepared baking sheet. Repeat with remaining puff pastry sheets and ingredients.
Brush the tops of the Wellingtons with egg wash, and cut vents with the tip of a sharp knife.
Bake until puffed and golden, about 15 minutes. Let rest for 5 minutes. Serve with Dill-and-Caper Creme Fraiche Sauce.
Dill-and-Caper Creme Fraiche Sauce:
In a small bowl, whisk to combine the creme fraiche, cream, dill, capers and some salt and pepper.