Preheat the oven to gas 7, 220ºC, fan 200ºC.
Put the lamb in a roasting tin in which it fits comfortably. Mix the 2 tbsp lemon juice and zest of 1 lemon with half the garlic and rub it over the lamb skin. Season well.
Put in the oven for 20 minutes then turn the heat down to gas 4, 180ºC, fan 160ºC and roast for 1 hour 10 minutes. Take out of the oven and allow to rest for 20 minutes, covered tightly with foil and a tea towel.
To make the salsa verde, whizz the shallot, mint, parsley, capers and anchovy together in a food processor. Add the remaining lemon zest and juice and garlic plus enough olive oil to make a spoonable dressing. Stir in the mustard.
Serve with the lamb, spooning some over each portion as you carve, with some roast baby new potatoes on the side.