Grind the salt, sugar, thyme sprigs and peppercorns in a pestle and mortar. Scatter half over the base of a shallow dish, lay the mackerel fillets on top and cover with the rest of the cure. Leave for a couple of hours for a light cure, or for up to twelve hours. Rinse well under running water, then skin the mackerel and slice, thinly or thickly, as you like.
«You can adjust the ratio of salt to sugar as you wish - some like more salt, some more sugar.»