If necessary, defrost the mackerel, remove the entrails, fins, head and tail. Wash and dry, then cut into boneless fillets. Peel the red onion and cut into half rings. Wash the dill, dry and finely chop. Prepare sterilized jars and lids.
Prepare mackerel for marinating. Cut the mackerel into pieces. Place mackerel in layers in a jar, then onions, and repeat until the very top of the jar. The last layer should be the onion.
Marinate the mackerel. Dissolve salt in cold water, add peppercorns, pour vinegar and sunflower oil. Pour the resulting marinade over the mackerel, close the jars with lids and turn over several times. Place the marinating jars in the refrigerator for 2 hours.
Place the finished marinated mackerel with onion and vinegar in a bowl and sprinkle with dill.