Arrange 6 oysters on the half shell in a vegetable steamer. Combine vinegar and mirin in a small bowl, and stir well. Drizzle 1 teaspoon vinegar mixture over each oyster; top each with 1 cilantro sprig, 1 slice of ginger, and 1 strip of chile. Sprinkle half of crushed peppercorns over oysters. Steam, covered, 2 minutes or until edges of oysters curl. Repeat procedure with remaining oysters, vinegar mixture, cilantro, ginger, chile, and peppercorns. Serve warm.