Arrange onions in layers in a large shallow bowl. Combine oil, saffron, ginger, pepper, half cinnamon & sugar. Pour over onions, add just enough water to cover. Mix well & let marinate for 2 hours.
Place celery stalks crossways in the bottom of a tagine or casserole. This protects the onions from scorching. Arrange onions in overlapping layers & pour over marinade. Sprinkle with remaining cinnamon & sugar.
Cover tagine & cook at 325F for 30 to 40 minutes & then uncover & brown under a broiler just before serving. The onions should be lsightly caramelized & very tender.