1. Put the butter into a medium bowl and microwave for about 15 seconds, until some of the butter has melted but the rest is soft but not melted
2. Into the bowl add the rest of the marinade ingredients: salt, pepper, lemon juice and lemon rind. Stir well until the unmelted butter has blended with the lemon juice and rind. Set aside
5. With a paring knife (or small sharp knife) make small incisions in the redder part of the surface deep enough to be able to insert a wedge of garlic
6. Into each hole place a wedge of garlic and a few rosemary leaves. Repeat every 5 cm or 2 inches over the entire surface of the lamb
7. Place lamb onto the raised rack inside the roasting pot and with a basting brush coat with the lemon butter marinade. Repeat coating until all the marinade has been applied (it is all right if marinade drips to the bottom)
8. The lemon zest will be at the bottom of the bowl, spread it evenly over the entire surface including the cut part of the leg with the exposed meat
9. Place uncovered into the middle of the preheated oven and roast for 20 minutes
10. Reduce the oven temperature to 180 C (360 F) and continue roasting for about 1 hour and 15 to 20 minutes for medium doneness (actual time will depend on your leg of lamb weight and thickness). A bit longer for well done. If using a meat thermometer, insert into the middle of the thickest part of the meat making sure not to hit the bone.
11. The internal temperature readings: Medium rare 55 C (130 F). Medium 65 C (150 F).
12. When the internal temperature has reached the desired reading, transfer the leg of lamb to a plate and let rest. NOTE: while resting, the internal temperature will continue to rise and further roast the meat, so do not over-roast because the meat will dry out. Cover with a tent of aluminum foil and let rest.
13. Stove top method: place the roasting pot on the stove top over low heat. Add the water and apple cider vinegar. Stir to deglaze the drippings at the bottom of the pan. Continue cooking and stirring until the liquid has reduced by about 1/3.
14. Pour sauce into gravy bowl and set aside until ready to serve
15. Present the entire leg on a platter and carve at the table, or pre-cut and serve the slices on a large serving plate. Enjoy!