Place the lamb loin in a dish and cover with the combined garlic, lemon juice and olive oil then let marinade for 3 hours.
Prepare the crumb by combining the breadcrumbs, parsley, garlic, lemon zest and olive oil. Mix well.
Preheat the oven to 160 degrees C. Drain the lamb then cook uncovered for 40 minutes.
Press the crumb on the top of the lamb and continue to cook until cooked to your preferred doneness, about 30 minutes. Rest for 10 minutes then serve with mashed potato and asparagus, or other seasonal vegetables.