Combine the vinegar, salt, peppercorns and cardamom pods in a saucepan. Warm over a medium heat, stirring to help dissolve the salt.
Add the lime and onion slices and let the mixture cool to room temperature. Arrange the herring fillets in a large sterilised glass jar.
Pour the vinegar mixture into the jar and seal well.
Chill for at least 2 weeks before serving.
*Some Kosher products may be available in limited stores.
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