Score the skin on the lamb rack with a sharp knife on the diagonal. Mix the mustard with the honey and spread all over the lamb except bones.
Strip the leaves off the rosemary and chop really fine, almost to a powder. Grate lemon zest and the parmesan and whizz it with the rosemary and the breadcrumbs in a blender or food processor. You can just mix it all together in a bowl but whizzing breaks up the breadcrumbs and makes for a more uniform coating. Tip onto a shallow dish.
Dip the lamb rack in the herb and parmesan crumbs making sure the coating sticks well, press it on with your fingers.
Preheat the oven to 240C/475F/gas 9. Place the lamb rack on a baking tray, skin side up and roast for 10 minutes. Turn the heat down to 180C/350F/gas 4 and roast for further 20-25 minutes (roasting time after the initial 10 minute blast should be 15 minutes per pound, provided your lamb had been brought to room temperature). Let it rest in a warm place for at least 10 minutes.
