Preheat the oven to 170°C.
Put the pistachios, herbs, lemon juice and oil in in a small food processor and whiz until you get a coarse paste. Add the lemon zest and season.
Unroll the lamb (if deboned/rolled) on a chopping board and pierce all over with a small sharp knife. Insert the garlic slices into the insertions. Using most of the pistachio mixture, stuff the lamb and roll it back into its original shape.
Rub the skin with the olive oil and season. Put it in a roasting dish, then roast in the middle of the oven for 30 minutes.
Take it out of the oven and cover the dish with foil. Reduce the heat to 160°C and roast in the oven about 60 minutes.
Remove the lamb from the oven. Press the leftover pistachio mixture on top of the lamb to create a crust. (If you’re using lamb with the bone in- cover the lamb all over with all of the mixture) Baste with the roasting jus and bake for 15 minutes or until the crust is golden brown.
Remove the lamb from the oven and transfer it to a plate to rest for 10 minutes.
Meanwhile, make gravy from the roasting jus.
Carve the lamb and serve with the gravy and the stuffing/crust.