How to Make It
Sprinkle the salmon fillet with 1 teaspoon of salt. Cover the salmon and refrigerate for 1 1/2 hours.
Cut the fish crosswise into 8 even slices about 3/4 inch thick. Lay 2 of the salmon slices on their sides, matching the wide inner section with the thin belly section so they fit together. Tie the pieces together with kitchen string around the circumference to hold them together. Repeat with the remaining salmon pieces; cover and refrigerate.
Preheat the oven to 375°. In a small baking dish, drizzle the chicken and garlic cloves with oil. Season with salt and pepper, cover with foil and bake for 30 minutes, or until the garlic is softened. Reduce the oven temperature to 325°.
In a medium saucepan, heat the olive oil. Add the salmon bones, season with salt and pepper and cook over moderate heat, stirring, until the bones turn opaque, about 4 minutes. Add the roasted chicken and garlic with any accumulated juices, then pour in the stock. Bring to a boil. Simmer over low heat, skimming until reduced by half, about 40 minutes. Strain the reduction.
In a small saucepan, melt 2 tablespoons of the butter. Add the shallot and white peppercorns and cook over low heat until softened, about 5 minutes. Add the white wine, vinegar and tarragon sprig and boil over moderate heat until almost evaporated, about 12 minutes. Add the reduction and simmer over low heat until reduced to 2/3 cup, about 10 minutes; let the sauce stand off the heat for 1 minute.
In a small skillet, melt the remaining 2 tablespoons of butter until nutty brown. Whisk the brown butter into the reduced sauce, season with salt and pepper and stir in the chopped tarragon.
Set an 18-by-12-inch piece of parchment paper on a rimmed baking sheet and arrange the salmon on the paper. Cover with another 18-by-12-inch piece of parchment, fold the ends neatly under on all sides and press well to seal. Bake for about 12 minutes, or until the salmon is barely cooked through.
Reheat the tarragon sauce over moderate heat, whisking constantly; do not boil. Set each salmon fillet on a plate, spoon the sauce on top and serve.