Prepare avegetable stock from 2 onions, two cloves of garlic, a few carrots, a bulb of fennel, a small leak, some celery greens. Cut everything into chunky pieces and briefly fry in a little olive oil. Add two bay leaves, two or three twigs of fresh thyme, a good portion of black pepper, two crushed juniper berries and a little salt and stir, then add 1500ml (or 3 pt) or cold water.
Bring to the boil, reduce the heat and let simmer for 10 minutes.
Meanwhile, clean two Mackerels if the fishmonger hasn’t done it for you: remove the heads and the innards, and rinse.
Take the stock off the heat, wait until it stops bubbling. Submerge the mackeles, put the lid back on and let slowly cool down in a corner of your kitchen (not in a particularily hot or cold spot).