Start out with your lamb at or near room temperature as this will help to ensure a more even cook throughout.
Preheat the oven to 200 C / Gas 6. Line a baking tray or roasting tin with aluminium foil.
In a bowl, combine 3 tablespoons oil with the the rosemary, garlic and 2 tablespoons mint sauce. Mix well then rub evenly over the racks of lamb. Season generously.
Heat the remainder of the oil in a large frying pan over high heat. Place lamb in frying pan when the oil is very hot. Brown well on all sides, about 5 to 8 minutes in total. Remove from heat and immediately transfer lamb to prepared roasting tin. Brush the remainder of the mint sauce over the top of the rack and sprinkle with fresh mint and parsley.
Roast in the preheated oven for 20 to 25 minutes for medium, cook in increments of 10 minutes for a more well done lamb. Remove from the oven, transfer to a plate and let rest 10 minutes before slicing.
For the best flavours, rub the lamb in oil, herbs and garlic several hours prior to cooking.