Combine the lemon zest, lemon juice, garlic, spices, herbs, and olive oil in a mini food processor. Add 1-1/2 teaspoon of salt and a good amount of freshly ground black pepper. Process on and off until a rough paste forms, set aside.
In a large bowl, place the leg of lamb and poke the meat all over about 3 times with a small sharp knife. Rub and massage the spice paste all over the meat. Cover with plastic wrap and refrigerate overnight.
The next day, preheat the oven to 375 degrees F.
In a 12" X 16" X 2" baking dish, transfer the lamb with the marinade. Cover the dish tightly with foil and roast for one hour.
Reduce the oven temperature to 350 degrees F. Add the celery root, carrots, and garlic to the baking dish. Contunue to roast for 4 hours, basting the meat and vegetables every hour.
Remove the foil and cook for another 30 minutes, or until lamb is browned and falling apart.