Make incisions into the lamb and place a sprig of rosemary and garlic in each hole. Rub with olive oil and season well. Drizzle with lemon juice and keep in fridge for at least 3-4 hours, over night is best. Preheat the oven to 180°C. Heat a frying pan to high heat and add the oil. Sear the lamb for 2 to 3 minutes , browning all sides.
Then place the lamb onto the bones that you kept from the de-boning and place into the oven with the mint, cover with foil and slow roast for approximately 90 minutes minutes, or if you like your meat rare, 20-25 minutes. The cooking time of course will depend on the size of the lamb loin too.
Once ready, cover loosely with aluminum foil and allow to rest for 10 to 15 minutes. Now take the meat sauce, bones and mint that was left in the bottom of the roasting pan and pour in a pot and start reducing the sauce. Add some water if there is too little liquid. Add the red wine and keep on reducing. When the sauce is the right consistency, beat the butter in a knob at a time. You want a glossy dark brown jous.
When ready to serve, remove the string from the lamb and slice. You can cut it in thin slices and present it on a meat platter or for nicer presentation, slice off the very ends and then cut the loin into four, six or eight equal pieces, depending on how big your loin is.