Peel The Onions, roughly dice them and set aside. Remove and discard celery root tops. Using a paring knife, peel and discard the thick black outer layer. Cut the celery root into pieces large enough to fit in the hopper of a food processor. Fit with a shredding blade and shred the celery root. Combine onion and celery root in a heavy pot. Cover onion and celery root and place over low heat. Cook for 20 minutes, stirring frequently. Remove cover and cook, stirring, another 10 minutes or so or until the mixture is dry. Keep warm while preparing the rest of the recipe. Place mushrooms, shallots and 1/4 teaspoon salt in a food processor and puree until smooth. Scrape this mixture into a medium saucepan over medium heat and cook, uncovered, stirring, until the moisture has evaporated and the mixture is dry. Remove from the heat and hold in a hot oven until salmon is done. Preheat the broiler. Butter a 9-inch baking dish. While mushroom mixture is cooking, combine the wine, garlic and 1/4 teaspoon salt in another medium saucepan over medium heat. Cook, uncovered, until reduced by half. Add the fish stock and reduce again until the mixture starts to thicken. Add cream and reduce until sauce will coat the back of a spoon. Scrape into a blender, add the parsley and blend until smooth. Replace in saucepan. Place the salmon in the baking dish and coat the surface with melted butter. Sprinkle with 1/4 teaspoon of salt, and place under broiler for 5 minutes. Replace parsley cream over medium heat on top of the stove and whisk in the unsalted butter. To serve, place dollops of celery root and mushroom puree on a warm plate. Lay a piece of salmon on top between the 2 and spoon some sauce over the top.