Run your hand lightly over the salmon, and if you feel any little pin bones sticking up, pull them out with tweezers or small pliers.
Cut the salmon into 6 equal portions.
Place the horseradish and apple in a food processor until finely grated. Pour in the vinegar through the feed tube, process briefly, then season with salt and set aside with the cover partly ajar to disperse the fumes until ready to serve.
Heat the oil in a heavy skillet. Place the salmon pieces in the skillet, skin side down, and saute 4 to 6 minutes, until the skin is very crisp but the top 1/2 inch (1.3 cm) of the fish still looks uncooked.
Cover the pan for a minute, uncover, and transfer the fish, skin side up, to 6 warm dinner plates.
Spoon some of the horseradish mixture alongside and serve.