To make the sauce: In a bowl stir together the shallot, the vinegar, the salt, the sugar, the pepper, the oil and the horseradish and chill the sauce, covered until serving time. The sauce may be prepared up to this point 1 day in advance and kept covered and chilled. Just before serving stir in the parsley. Serve with oysters and garnish with seaweed and lemon wedges.;