Preheat oven to 425 degrees F. Slit lamb and insert garlic into slits. Brush 2 tbs of olive oil onto lamb. Mix 2 tbs thyme, the dried rosemary, 2 tbs pepper, and coriander in a small bowl. Place lamb in shallow roasting pan. Cover outside of lamb with herb crust mixture|
Mix the potatoes in a large mixing bowl with 3 tbs olive oil, fresh rosemary, 2 tsp thyme, and 1 tsp pepper, and course salt. Place potato mixture in roasting pan around lamb.
Roast lamb in center of oven for 45 minutes at 425 degrees F. Reduce heat to 375 degrees F and stir the potatoes to keep them from sticking. Cook for 30 more minutes.
Remove lamb from oven. Insert instant read meat thermometer. Temperature should be about 120 degrees F for rare. Let the lamb rest for 15 minutes before carving. While it is resting, the meat will cook and the temperature will rise to 135 or 140 degrees F.
Serve lamb and potatoes on platter garnished with fresh rosemary sprigs. Serve with mint jelly.