- In a large bowl, combine all the marinade ingredients. Add the lamb, cover and marinate in the refrigerator for 24 hours.
- With the rack in the middle position, preheat the oven to 200 ° C (400 ° F).
- Remove the lamb from the marinade. Keep half the marinade and discard the remaining. Place the rosemary on the inside of the leg of lamb. Close and tie the leg before placing it in an ovenproof baking dish. Pour the reserved marinade over the lamb. Season with salt and pepper.
- Bake for 15 minutes, then reduce the oven’s temperature to 190 °C (375 °F). Continue cooking for about 25 minutes. Remove the roast when the meat thermometer reads 63 °C (145 °F).
- Remove the lamb from the pan. Wrap with aluminum foil and place on a cutting board. Let rest for 10 to 15 minutes. The temperature should rise to about 68 °C (155 °F) for medium-rare meat.
- Strain the cooking juices through a sieve. Set aside.
- In a skillet, sauté the shallots in the butter. Add the roast cooking juices and reduce until syrupy. Add the demi-glace and cook for 1 minute. Adjust the seasoning.
- Slice the meat and coat with the sauce. Serve with Roasted Winter Root Vegetables (see recipe).