Layer onions, carrots, garlic, bay leaves and thyme on bottom of nonreactive covered roaster large enough to hold lamb.
Place lamb on top of onion and carrot mixture. Roast, uncovered, at 425 degrees for 30 minutes. Remove roaster from oven. Generously season lamb to taste with salt and.
pepper. Return to oven and roast another 30 minutes.
Remove roaster from oven, leaving oven on. Place roaster on top of stove, slowly pour wine over lamb, cover, and bring liquid to a boil.
Return roaster, covered, to oven. Roast 4 to 5 additional hours until lamb is fork tender, but not falling off bone. (Timing will vary according to size and age of lamb and type of roasting pan used,).
Check on lamb, reducing oven temps if lamb begins to burn or liquid begins to evaporate too much. When lamb is fork tender, bury potatoes and tomatoes in liquid. Cover and roast until potatoes are cooked through, about 1 hour more. Lamb should now be very tender, still juicy and falling off bones.