This is an interesting way to cook the lamb and a vegetable gratin at the same time. The juices from the lamb drip into the gratin dish placed underneath the lamb. It's good, and a fun way to cook, but beware, it can get rather messy.
Split 1 clove garlic. Rub bottom of 16x10x2 inch oval gratin dish with split clove of garlic. Arrange potatoes over bottom of pan. Chop together remaining 5 garlic cloves, and thyme. Season generously to taste with salt, pepper and 1/3 of garlic-thyme combination. Layer sliced onions on top. Season to taste with salt, pepper and another 1/3 of garlic and thyme. Layer peppers on top of onions. Season to taste with salt, pepper and remaining 1/3 garlic and thyme. Pour over wine, then oil.
Trim excess fat from lamb. Season meat to taste with salt and pepper. Place sturdy oven rack directly on top of gratin dish. Set lamb on rack so that juices will drip into gratin.
Roast, uncovered, at 400ºF about 1 hour and 15 minutes for rare. Turn lamb every 15 minutes, basting with liquid from dish underneath. Remove from oven and let lamb stand on rack so that juices will drip into gratin.
To serve, carve lamb into thin slices and arrange on warmed dinner plates or on serving platter with vegetable gratin alongside.