Drizzle apple and horseradish dressing over oysters for a sophisticated seafood starter to share. Serve on a bed of ice or rock salt with sprigs of dill Mix the vinegar and shallots for the dressing in a small bowl and leave this to sit, covered, for an hour or so, so that the shallots can infuse the vinegar. Add all the other ingredients, stir and put into a serving bowl. Shuck the oysters – make sure you have an oyster knife; don’t try to do this with a regular kitchen knife. Rinse the oysters under cold running water. Throw away any that are open and don’t close if you tap them against the side of the sink. Place an oyster rounded-side up on a work surface. Grip the oyster with a tea towel to help protect your hand, leaving the narrow hinged end exposed. Place the tip of the knife between the top and bottom shells just next to the hinge. Carefully push it into the shell, twisting and wiggling the knife tip, to release the top shell. At first, it may seem like you aren’t getting there, but keep going with gentle pressure until the shell pops open. Try to keep the oyster level, so the delicious liquor (the briny, salty sea water) stays inside the deeper bottom shell. Wipe your knife, then pry open the shell by inserting the knife tip in a few other spots, twisting it to release the shell completely. Keeping the oyster level, run your knife along the inside of the upper shell to cut the muscle that attaches the oyster to it. Remove the top shell. Run your knife along the inside of the lower shell and gently cut the oyster free. Leave the oyster nestled in its shell. (If you open an oyster that has a strong, sulphurous smell, chuck it out: it’s dead.)
Transfer the oyster in its bottom shell to a bed of crushed ice or rock salt that will keep the shell level, while you repeat the process with the remaining oysters. Serve immediately with the dressing; each person should use about ¼ tsp of the dressing for each oyster (you can dress each one in advance yourself if you prefer, and add a dill sprig to the tops).