1. In a large bowl, combine the cut lamb with the olive oil, crushed herb sprigs and garlic cloves. Place the lamb in a non-reactive dish and marinate covered overnight or for up to two days.
2. Scrub the skins of the potatoes well to remove any dirt. Place them in a pot of cold water and bring it to a boil. Add 3 tablespoons of salt, reduce the heat to a simmer and cook for 12 to 15 minutes until the potatoes are tender. Drain the potatoes and chill over night in the refrigerator.
3. Preheat the grill or broiler.
4. Remove the lamb from the marinade and brush off the herbs and garlic. Divide the lamb among 4 long kebab skewers or wooden skewers that have been soaked in water for 30 minutes. Place 3 potatoes, 3 mushrooms, 3 onions and 3 tomatoes on each skewer spaced equally with the 4 cubes of lamb. Start and end with a lamb cube. Season the kebabs with salt and pepper.
5. Grill the kebabs over medium heat or broil for 7 minutes on each side for medium/medium rare. Let the lamb rest for 5 minutes before serving. Serve with tapenade on the side.