How to Make It
In a large nonreactive bowl, massage the daikon with the grey salt until wet. Pack the daikon in the bowl and press plastic wrap directly on top. Cover with a similar bowl to submerge the daikon in the brine. Let stand in a cool place for 5 days. Drain and rinse in cold water; drain. Refrigerate.
In a small saucepan, combine 3/4 cup of the wine with both vinegars, the shallot and ginger. Boil over moderately high heat until the liquid is almost evaporated, about 15 minutes. Add the cream and simmer over moderate heat until reduced by one-third, about 5 minutes. Strain through a fine-mesh sieve into a bowl, pressing on the solids; discard the solids. Return the liquid to the pan. Off the heat, gradually whisk in the butter. Season the beurre blanc with fine sea salt and keep warm.
Season the fish with fine sea salt. In a steamer basket set over a medium skillet with 1 inch of simmering water, steam the fish, skin side down and covered, over moderately low heat, until just cooked through, about 8 minutes.
Meanwhile, in a medium saucepan, warm the drained daikon with the remaining 1/2 cup of wine over moderately low heat, tossing occasionally. Arrange the daikon on plates. Top with the fish and spoon the beurre blanc on top. Garnish each serving with a sorrel leaf and black pepper.