Cut slits all over the lamb with the tip of a sharp knife and insert the garlic slivers into them.
Brush the lamb all over with 2 tbsp of the olive oil.
Combine 2 tbsp thyme, 2 tbsp pepper and the coriander in a small bowl and mix well
Pat herb mixture all over the lamb to form a crust. Place the lamb in a shallow roasting pan.
Put the potatoes in a large mixing bowl. Add remaining olive oil, remaining thyme, remaining pepper, chopped rosemary, and salt. Toss to mix well and arrange around lamb in roasting pan.
Put pan in oven and roast for 45 minutes. Then, reduce heat to 375 and cook at least 30 minutes more.
Remove pan from the oven when internal temperature is at least 120 F (for rare). Cover with tin foil and let sit for 15 minutes before carving.
NOTE, for rare lamb, allow 12 minutes per pound. For well done, 18 minutes per pound.