Mix together all the ingredients for the sumac yoghurt sauce and keep in the fridge until needed.
Put the pistachios into the small bowl of a food processor. Blitz for a few seconds, to roughly chop, then put into a medium bowl.
Add the rocket to the processor, blitz for a few seconds to roughly chop, then add to the bowl of pistachios.
Continue with the onion and garlic, to form a smooth paste, and add to the bowl.
Add the lamb, 1 tablespoon of oil, ¾ teaspoon of salt and a good grind of pepper. Mix well to combine.
With wet hands, shape the mix into about 20 patties, each about 5cm wide, 2cm thick and weighing about 40g.
Put 1 tablespoon of oil into a large non-stick frying pan and place on a medium heat.
Once hot, add the patties – as many as you can fit without overcrowding the pan – and cook for 7 minutes, turning once halfway, until golden-brown and just cooked through.
Keep warm while you repeat with the remaining patties, adding another tablespoon of oil to the pan if needed.
When all cooked, pile the patties on to a large platter and serve with the sumac yoghurt alongside.
