Step by step recipe:

1. 2 min.
   Have your fishmonger clean and remove the heads from the mackerel.
   
   Wash them thoroughly.
2. 7 min.
   Put the fish into a pan of cold salted water and put this on low heat.
   
   Cook the mackerel at a gentle simmer. Ideally, use an thermometer stuck into the middle of the fish and stop cooking when the temperature reaches 140°F (60°C).
   
   If you don't have a thermometer, cook for 7 minutes, counting from when the water starts to simmer.
3. 15 min.
   Remove the mackerel skin and discard this along with the bones.
   
   There's no doubt about it, this is the longest and least pleasant part of the recipe; mackerel have a lot of bones.
   
   Take care to pick out and discard the dark flesh from along the top of the fillets. Only keep the best of the flesh, the white parts.
4. 2 min.
   Mix in a bowl: 2 glasses dry white wine, 1 glass white (spirit) vinegar, 1 tablespoon French mustard, salt and pepper.
5. 2 hours
   Immerse the mackerel fillets in this mixture, cover with plastic film and leave to marinate for at least 2 hours.
6. 
   Your mackerel fillets are ready. Take them out and drain them as and when required, otherwise they are better kept in the marinade.
   
   Personally, I love eating these fillets on a slice of toast that has been rubbed with a piece of onion.