Preheat oven to 180 degrees C. Rinse the salmon under cold water and place in a deep oven pan to fit snugly. Place water, white wine, lemon, bay leaves, peppercorns and salt in a saucepan and bring to a boil. Pour over the salmon so that it is covered, adding more boiling water if necessary.
Cover pan with foil and poach in the oven for 8 minutes per 500g salmon (32 minutes for a 2 kg fish). You can also do this on the stovetop but poach very gently so the liquid is not quite boiling, just trembling.
Meanwhile, to make the sauce, simmer the vinegar and bay leave in a small saucepan until reduced by half. Remove the bay leave and set liquid aside. Melt the butter in a separate saucepan. Set aside and let cool to room temperature.
Place the egg yolks, lemon juice and salt in food processor and process until frothy and pale. With the motor running, pour in butter alternating with reduced vinegar in a thin, steady stream. Add the tarragon and process to combine. Add salt if desired. Keep warm until serving or the sauce will set. Makes 1 cup.
Carefully remove the fish from the liquid and transfer it to a serving platter. Remove the skin from the gills to the tail. Serve with bearnaise sauce on the side.