1. Preheat the oven to 400°F. Rub the bottom of a large (16x10x2-inch) gratin dish with the split garlic clove. Arrange the potatoes in a single layer. Season with salt, pepper and some of the thyme and chopped garlic. Layer the sliced onions on top; repeat the seasonings. Layer the tomatoes on top and season with salt, pepper and remaining garlic and thyme. Drizzle the top with the wine and then the oil. 2. Trim the thicker portions of fat from the leg of lamb. Season the meat with salt and pepper. Place the lamb on a sturdy cake or oven rack directly on top of the gratin dish. 3. Roast, uncovered, for about 1 hour and 15 minutes for rare lamb. (For well-done, roast 30-40 minutes more.) Turn lamb every 15 minutes, basting with liquid from the dish underneath. Remove from the oven and let the lamb sit for 20 minutes before carving. 4. To serve, carve the lamb into thin slices and arrange on a serving platter, with the vegetable gratin alongside.