Preheat oven to 425°F.
Heat a large non-stick skillet over medium heat. Add oil, onions and mushrooms. Season with salt and pepper, and sauté 10 -15 minutes or until the liquid from the mushrooms has evaporated. Remove from heat and add to a medium-sized mixing bowl. Add dill, parsley, lemon juice, mustard, hard-boiled egg, salt and pepper. Mix to combine.
Wipe out the pan you used for the mushrooms and return it to medium-high heat with a drizzle of EVOO. Season salmon with salt and pepper, and sear 1-2 minutes on each side in a drizzle of oil. Remove skillet from the heat.
Spread the puff pastry sheet onto a counter and roll to 1/8 of an inch thick. Cut in half.