Preheat oven to 400 degrees.
Make 4 slits in front of lamb and 4 slits in back (spacing them apart) with the tip of a paring knife. Insert a whole garlic clove in each slit. Rub butter all over the lamb with your hands.
Combine salt, pepper and rosemary and press into lamb on all sides. Place on rack in roasting pan large enough to hold meat with a little extra space (but not too much!) on the sides. Pour water into the bottom of pan. Roast 20-23 minutes a pound for medium-rare. Remove pan from oven, place roast on a cutting board and cover with foil. Let rest for 10-15 minutes, allowing juices to redistribute throughout the meat. In the meantime, deglaze roasting pan on top of stove by setting atop a medium flame and pouring wine in. Let juice reduce and add chicken stock, salt and pepper. Reduce to half, about 6-8 minutes, strain and pour into a saucier to be served on the side of sliced meat.