Strain out onion and spices. Cool onion and spices, and sauce, separately in the refrigerator. Cut cured herring into 2 fillets, removing the backbone. Roll each fillet around a small piece of dill pickle and fasten with a toothpick. Scatter some cooled onions and spices on bottom of a large container.
Pack rollmops on end in layers in the container, adding some onions and spices between layers.
Cover rollmops with cooled sauce. Store in the refrigerator for 2-3 days before using. Keep refrigerated . Wash pieces in cold water, drain, and dredge in salt. Rinse off salt in cold running water.