Put the steaks on a board and cover with a sheet of clingfilm. Using a mallet or the base of small, heavy saucepan, bash the steaks until they are about 1.5cm thick. Cut up the steaks, fat and all, into 1.5cm cubes and put in a bowl with the ground black lime, lemon zest and juice, cumin, garlic, two tablespoons of olive oil, a teaspoon and a half of salt and a good grind of pepper. Leave to marinate at room temperature for at least two hours (or refrigerated overnight).
For the sumac onions, mix the onion, sumac, vinegar and an eighth of a teaspoon of salt, and leave to soften for at least 30 minutes.
Thread the beef on to the skewers, packing the cubes closely together. Generously grease a grill pan with sunflower oil and put on a high heat. Once smoking, lay in the skewers in batches and cook, turning as you go, for about three to four minutes in total, until charred all over but not over-cooked. Transfer to a tray and cover with foil to keep warm.
Warm the bread on a hot grill for about 30 seconds on each side.
To serve, put a flatbread on each plate and top with two beef skewers each, brushing them with the remaining tablespoon of olive oil and sprinkling with the chilli. Toss the parsley and mint into the sumac onions, divide the mixture between the plates and serve.
