Pre heat your oven to 180 deg C. Wash your lamb loin using cold water and then dry using paper towels.
Combine the pistachio kernels, lemon juice and zest, garlic, mustard and parsley in a food processor and with the motor running add your olive oil in a thin stream. Process until it all comes together and forms a paste.
Lay the loin of lamb on a baking tray flesh side up and proceed to place the stuffing in the centre. Roll it up gently and tie it up with kitchen string. Drizzle a little olive oil over the tied up lamb loin and add some more salt and pepper.
Cook in the oven for the first 10 mins at 180 deg C and then drop the temperature down to 160 deg C. Check on the lamb 20 mins later. If it appears a little dry add half a glass of water. Cook for a further 20-25 mins. (Total cooking time is anywhere between 45-60 mins)
Remove from the pan and lay it on a plate to rest covered in foil. Slice and serve immediately.