Add the roughly chopped white onion, 5 garlic cloves, the juice of 1½ lemons, 2 tbsp of olive oil and 1 tbsp of water to a food processor. Blend until completely smooth, with no visible pieces of onion or garlic, adding the second tablespoon of water if the blades struggle. Add the 2 tsp of cumin, 1 tsp of paprika, 1 tsp of za’atar, ½ tsp of cinnamon, the pinch of cloves, 10g of the salt and a generous grind of black pepper, then blend again until uniform. Taste it before any meat goes in: it should be noticeably saltier than anything you would eat directly, sharp with lemon and warmly spiced. If it only tastes pleasantly seasoned, add the remaining 2g of salt and blend again. That aggressive seasoning is correct because only a thin film of marinade clings to each cube, and that film has to season the whole piece.

Trim the 800g of lamb shoulder of silverskin — the thin, white, opaque membrane on the outside that never renders and stays chewy — and any hard pieces of connective tissue, but leave the intramuscular fat and some of the outer fat in place. Cut the trimmed lamb into uniform 3cm cubes so every piece cooks at the same rate. Put the cubes in a large bowl, pour over the marinade and mix thoroughly with your hands, turning and pressing each piece until every surface is coated. Cover tightly and refrigerate for at least 90 minutes and no more than 4 hours. Wash your hands, board and knife well after handling the raw lamb. That firm time limit matters because the lemon juice keeps working on the surface proteins, and beyond 4 hours it turns the outside of each cube soft and mushy instead of pleasantly tender.

While the lamb marinates, cut the 2 green bell peppers into 3–4cm square pieces, roughly matching the size of the lamb cubes. Leave the 400g of cherry tomatoes whole. Take the vegetables out of the fridge before threading so they are not fridge-cold, but keep the lamb refrigerated until the moment you thread it. If using wooden skewers, soak 12 of them in water now. Toward the end of the marinating time, light a charcoal grill or wood fire, or heat a gas grill, to high. That size matching and the warmer vegetables are what keep the peppers and tomatoes from slowing the cooking of the lamb pieces beside them.

Divide the lamb, peppers and tomatoes evenly across the 12 skewers, threading them alternately — lamb, pepper, tomato, lamb, pepper, tomato — and stopping about 2cm from the tip. Let excess marinade drip back into the bowl as you go, then discard what is left in the bowl, since it has been in contact with raw meat. Leave a small, visible gap between each piece rather than pressing them together. If the cherry tomatoes are very small, thread two together in a section; larger ones go on singly. That spacing is what lets heat circulate along the whole skewer so every exposed surface chars instead of steaming against its neighbour, while the pepper and tomato juices baste the lamb beside them.

Make sure the grill is thoroughly preheated: the grate should be very hot before any skewer touches it, and a charcoal or wood fire gives the best flavour as fat and juices drip onto the coals and send up smoke. Lay the skewers over the direct heat and turn them every 2–3 minutes so each face of every lamb cube meets the fire. Grill for 10–14 minutes in total, depending on the heat and cube size, until the lamb has deep, slightly blackened edges on several sides, the tomatoes have blistered and started to burst, and the peppers are charred at the edges but still hold their shape. Check the thickest lamb cube on several skewers with an instant-read thermometer: it must read at least 63°C / 145°F. If the outside chars before the centre gets there, move the skewers to a cooler part of the grill and keep turning, because the thermometer decides, not the clock. Without a grill, heat a cast-iron or heavy stainless-steel pan over the highest heat for 3–4 minutes until smoking, add a little high-smoke-point oil and sear the skewers in batches of 3–4, turning every 60–90 seconds for 10–12 minutes until charred on several sides and at the same 63°C. That combination of real high heat and regular turning is what builds a deep char on every face while the fat-marbled shoulder stays juicy inside.

Transfer the skewers to a warm plate and rest them for 5 minutes — longer than the minimum 3-minute rest that whole-muscle lamb needs after reaching 63°C — so the juices settle back through each piece. Sprinkle generously with extra za’atar, scatter over fresh flat-leaf parsley and set lemon wedges alongside for squeezing. Serve 3 skewers per person with warm homemade pita or lavash flatbread and toum or tzatziki on the side, so each kebab can be slid off into the bread with a spoonful of sauce. That rest is what completes the safe cooking of the lamb and keeps its juices in the meat rather than on the plate.