1. Mix the lamb with the olive oil and garlic, then leave, covered, in the fridge overnight (optional).
2. Purée all the pesto ingredients, except the olive oil, in a food processor, finally adding the oil in a steady stream. Season, then set aside.
3. Thread the lamb on to skewers (if using wooden ones, soak first in water for 30 minutes), or long sprigs of fresh rosemary. Brush off any garlic, as it will burn, then season.
4. Cook on a griddle or frying pan over a high heat, turning often, for 8-10 minutes for rare, or longer if you prefer. Serve with dressing and herb salad.