COMBINE ONION, WINE, SALT, PEPPER and 2 tablespoons butter in a large pot, place over medium heat and cook for 5 minutes. Add bread crumbs. Cook, stirring constantly, 3 minutes. Transfer contents to bowl and cool. Add parsley, tarragon and thyme. Lay fillet on work surface and carefully butterfly it by cutting in half to give you 2 thin fillets. Lay down a piece of parchment paper larger than the -20ºC illets side by side. Brush it with oil. Lay the 2 pieces of salmon on the parchment head to tail, forming a large rectangular fillet of salmon. Spread cool stuffing evenly over the salmon and roll up the salmon cigarette style. Use the paper underneath to assist rolling tightly into a cylinder. Preheat oven to 180ºC . Place salmon roll in a baking pan and bake 35 minutes. Meanwhile, heat the sherry in a saucepan; cook, reducing by half. Remove from heat and whisk in the remaining butter. Unwrap the salmon from the paper and place on a platter. Pour the sauce over and serve immediately.