Cook onion in boiling water 2 minutes. Drain and plunge onion into ice water; drain. Place onion in a large glass bowl.
Combine the rice vinegar, 1 cup water, sugar, black peppercorns, whole cloves, mustard seeds, whole allspice, and salt in a medium saucepan; bring mixture to a boil. Cover, reduce heat, and simmer for 5 minutes. Pour the vinegar mixture over the sliced onion. Cool. Cover and refrigerate mixture overnight.