Place the vinegar, sugar and water in a saucepan and bring to a boil. When the sugar has dissolved, remove from the heat and allow to cool a little.
Peel the carrots, then use a mandolin to slice into thin ribbons.
Cut the cucumber in half and slice into ribbons, again, using the mandolin. Slice the tomatoes into 12 rounds. Pick the leaves from the thyme.
Separate the leaves from the baby gem lettuce, wash, then place in a salad spinner to dry.
Slice the bun in half, lengthways, and set aside.
Once the pickling liquor has cooled, pour over the carrot and cucumber ribbons.
To cook the Mackerel:
Place a grill pan on a medium heat. Brush the fish, on both sides, with olive oil then season with sumac, picked thyme leaves and salt.
When the grill is hot, place the fish skin side down and cook for approx. 2 minutes. You will notice that it will gradually turn from transparent to opaque during this time. When it is 3/4 done, gently turn over and cook for no more than 30 seconds. place on a warmed plate or tray.
Place 2 slices of tomato on the bread, followed by some lettuce leaves. Add the fish and top with the pickled vegetables. Finish with plenty of freshly squeezed lemon juice.
«The vegetables can be pickled in advance and kept for a week.»