Roughly chop the pistachios by pulsing them in a food processor. Tip them into a large bowl.
Blend the rocket, mint leaves, and onion to a paste in the food processor. Add this mixture to the pistachios.
Add the lamb mince, coriander, cumin, sumac, salt and pepper to the bowl and mix with your hands until all combined. Shape into 8 patties.
Heat the butter in a large frypan over a low-medium heat and cook the patties for approximately 10 minutes per side, until golden brown and just cooked through.
While the patties are cooking make the minted sumac yoghurt by mixing all of the ingredients together in a small bowl.
Serve the patties with a salad and the yoghurt on the side.