Preheat oven to 400°F. Using small sharp knife, cut 1-inch-deep, 1/2-inch-long slits all over lamb. Insert garlic slices into slits. Rub oil all over lamb. Mix thyme, savory, rosemary, salt and pepper in small bowl. Rub herb mixture all over lamb. Set aside.
Generously butter large roasting pan. Combine potatoes, onions and stock in large pot (stock will not cover vegetables). Bring to boil. Reduce heat, cover and simmer until potatoes are halfway tender, about 10 minutes. Transfer potato mixture to prepared pan; spread evenly in pan. Sprinkle with salt and pepper. Bake 10 minutes. Reduce oven temperature to 375°F. Place lamb in roasting pan atop potato mixture. Roast until thermometer inserted into thickest part of lamb registers 130°F. for medium-rare, about 1 hour 50 minutes. Remove from oven. Tent with foil and let stand 15 minutes.
Thinly slice lamb. Arrange lamb on large platter. Surround with potato mixture and any juices from pan and serve.