Complex sentence: Rollmops are pickled herring fillets, rolled (hence the name) into a cylindrical shape around slices of onion, pickled gherkin, or green olive with pimento.
Split into simpler sentences: Rollmops are pickled herring fillets, rolled (hence the name) into a cylindrical shape around a savoury filling. The filling is usually slices of onion, pickled gherkin, or green olive with pimento.