Place orange juice, white wine, olive oil, garlic, fresh thyme and rosemary and pepper into a blender; mix. Pour into a large resealable plastic bag and add the lamb. Coat meat with the marinade, squeeze out excess air and seal the bag. Marinate in the refrigerator for 8 hours or overnight.
Remove the lamb from the refrigerator at least 30 minutes before roasting. Pat dry with a paper towel. Generously salt and pepper both sides of the meat.
Preheat an oven to 220 degrees C. Arrange two racks in the oven - a middle rack to hold the lamb, and a lower rack to hold a roasting pan to catch the drippings. Place the empty roasting pan in the oven while the oven is preheating.
Arrange meat directly on middle rack, fattiest side up so while the lamb cooks the fat will melt into the meat. Position roasting pan underneath meat to catch the drippings. Roast for 30 minutes, and then reduce heat to 150 degrees C. Roast to desired degree of doneness, about 90 minutes for medium. Remove from oven, cover loosely with foil and allow to rest for 10 to 15 minutes.