Preheat the oven to 425 degrees . Sprinkle a little flour on a work surface and place the puff pastry on top. Using a sharp paring knife, cut out 4 fish shapes about 2 by 8 inches. Arrange on a parchment-lined baking sheet and brush with the egg wash. Bake until golden, 12 to 15 minutes.
In a medium skillet, heat 1 tablespoon EVOO over medium-high heat. Add the mushrooms and cook until softened, about 5 minutes. Add the leeks and cook until wilted, 2 to 3 minutes; season with salt and pepper. Stir in the wine for 1 minute, then stir in the cream and cook until reduced, about 5 minutes. Stir in the dill and turn the heat to low.
Season the salmon with the Old Bay Seasoning and salt and pepper. In another skillet, heat 1 tablespoon EVOO over medium-high heat. Add the salmon and cook, turning once, for 6 minutes.
Dress the watercress with the lemon juice and remaining 1 tablespoon EVOO; season with salt and pepper. Top the salmon with the sauce and puff pastry and serve with the watercress.