Trim any excess fat from the Lamb leg and cut into 1–1½-inch cubes for even cooking.
In a bowl, whisk together Extra-virgin olive oil, Lemon juice, finely grated or minced Onion, minced Garlic, Ground cumin, Sweet paprika, freshly ground Black pepper and Fine sea salt. Add the lamb cubes, toss to coat thoroughly, cover and refrigerate for at least 2 hours or preferably overnight.
Prepare the vegetables: halve the Tomatoes if large (or leave small ones whole), cut the Long green peppers into large pieces and pull the Red onion into wedges. Toss the vegetables with a little Olive oil and a pinch of Salt.
Thread the marinated lamb onto skewers, alternating with the prepared tomatoes, peppers and onion wedges so each skewer has a mix of meat and veg.
Preheat a grill or broiler until very hot. Cook the skewers, turning every few minutes, until the lamb is charred at the edges and cooked to your preferred doneness (about 8–12 minutes depending on heat and cube size). In the last minute of cooking, brush the skewers with a little melted Butter for glossy flavor.
Make a simple salad: thinly slice any remaining red onion and toss with chopped Flat-leaf parsley, a sprinkle of Ground sumac, a drizzle of extra-virgin olive oil and a squeeze of lemon, then season with Salt to taste.
Warm the Flatbreads briefly on the grill or in a hot pan. Serve the kebabs piled on the flatbread with the sumac-parsley salad, and finish with wedges of Lemon to squeeze over everything.
