Shuck the oysters and retain the juice and shells. Clean and dry the shells.
Mix together the horseradish, vinegar, crème fraîche and oyster juice in a bowl, and season with a good helping of freshly ground black pepper. Finely chop the oyster flesh and mix with the dressing.
Serve the tartare in the reserved oyster shells, and garnish with chopped chives, flakes of sea salt and