1) Pat the lamb dry and score the fat by making shallow cuts all over it with the tip of a small, sharp knife.
2) Place the garlic, parsley, mint, lemon zest and juice, salt, pepper and oil in a food processor and process until smooth. Rub the paste all over the lamb and place it in a lightly oiled roasting tray. Let stand at room temperature for 30 minutes.
3) Meanwhile, preheat the oven to 220°C / gas mark 7.
4) Roast the lamb in the middle of the oven for 20 minutes. Reduce the heat to 180°C / gas mark 4 and cook until an instant-read thermometer inserted 5cm into the thickest part of the meat (not touching bone) registers 60°C, about 1 3/4 hours.
5) Transfer the lamb to a cutting board and let stand for 15 to 25 minutes (the internal temperature will rise to about 64°C for medium-done).
Slice the meat against the grain and serve with pitta bread.