Combine garlic, rosemary, salt and pepper in a small bowl and rub over the entire roast.
Transfer to a rack fitted in a shallow roasting pan fat side up. Add water to the pan. Tent with foil. Roast for 1 hour. Remove foil and continue cooking for another hour and 5 minutes (about 25 minutes per pound). Now turn the broiler on; broil for 5 minutes. Flip over and broil for 5 minutes on the other side.
Transfer to a plate and tent with aluminum foil. Let roast rest for 15 minutes.
Snip butcher's twine and slice by making perpendicular slices to the bone.
For Mint Pesto:
Mash the garlic and salt together in a small dish. Place in the bowl of your food processor fitted with the large chopping blade. Add the mint, lemon zest and pepper. Secure the cover. Process until the mixture has a paste-like consistency, scraping the sides of the bowl with a spatula if necessary. Cover. Now with the motor running add the oil in a slow steady stream through the small feed tube, thickening the mixture. Cover to prevent oxidation until you're ready to serve with the lamb.
I like my lamb medium rare and checked several cooking charts to find out what would be the perfect temperature. They indicated to cook until 130℉-135℉ inside measured with a cooking thermometer.However, when I followed those instructions, my roast was still uncooked inside. After an hour and 40 minutes, the thermometer read 135℉, but I had to put the roast back in the oven for another 25 minutes for a total of 2 hours and 5 minutes.I didn't check the final temperature, but I can tell you that this was about 25 minutes per pound.